Table of contents

  1. 1 Introduction
  2. 2 Common infrastructure
  3. 3 Semantics, structure, and APIs of HTML documents
  4. 4 The elements of HTML
  5. 5 Microdata
  6. 6 User interaction
  7. 7 Loading web pages
  8. 8 Web application APIs
  9. 9 Communication
  10. 10 Web workers
  11. 11 Worklets
  12. 12 Web storage
  13. 13 The HTML syntax
  14. 14 The XML syntax
  15. 15 Rendering
  16. 16 Obsolete features
  17. 17 IANA considerations
  18. Index
  19. References
  20. Acknowledgments
  21. Intellectual property rights

Full table of contents

  1. 1 Introduction
    1. 1.1 Where does this specification fit?
    2. 1.2 Is this HTML5?
    3. 1.3 Background
    4. 1.4 Audience
    5. 1.5 Scope
    6. 1.6 History
    7. 1.7 Design notes
      1. 1.7.1 Serializability of script execution
      2. 1.7.2 Compliance with other specifications
      3. 1.7.3 Extensibility
    8. 1.8 HTML vs XML syntax
    9. 1.9 Structure of this specification
      1. 1.9.1 How to read this specification
      2. 1.9.2 Typographic conventions
    10. 1.10 A quick introduction to HTML
      1. 1.10.1 Writing secure applications with HTML
      2. 1.10.2 Common pitfalls to avoid when using the scripting APIs
      3. 1.10.3 How to catch mistakes when writing HTML: validators and conformance checkers
    11. 1.11 Conformance requirements for authors
      1. 1.11.1 Presentational markup
      2. 1.11.2 Syntax errors
      3. 1.11.3 Restrictions on content models and on attribute values
    12. 1.12 Suggested reading
  2. 2 Common infrastructure
    1. 2.1 Terminology
      1. 2.1.1 Parallelism
      2. 2.1.2 Resources
      3. 2.1.3 XML compatibility
      4. 2.1.4 DOM trees
      5. 2.1.5 Scripting
      6. 2.1.6 Plugins
      7. 2.1.7 Character encodings
      8. 2.1.8 Conformance classes
      9. 2.1.9 Dependencies
      10. 2.1.10 Extensibility
      11. 2.1.11 Interactions with XPath and XSLT
    2. 2.2 Policy-controlled features
    3. 2.3 Common microsyntaxes
      1. 2.3.1 Common parser idioms
      2. 2.3.2 Boolean attributes
      3. 2.3.3 Keywords and enumerated attributes
      4. 2.3.4 Numbers
        1. 2.3.4.1 Signed integers
        2. 2.3.4.2 Non-negative integers
        3. 2.3.4.3 Floating-point numbers
        4. 2.3.4.4 Percentages and lengths
        5. 2.3.4.5 Nonzero percentages and lengths
        6. 2.3.4.6 Lists of floating-point numbers
        7. 2.3.4.7 Lists of dimensions
      5. 2.3.5 Dates and times
        1. 2.3.5.1 Months
        2. 2.3.5.2 Dates
        3. 2.3.5.3 Yearless dates
        4. 2.3.5.4 Times
        5. 2.3.5.5 Local dates and times
        6. 2.3.5.6 Time zones
        7. 2.3.5.7 Global dates and times
        8. 2.3.5.8 Weeks
        9. 2.3.5.9 Durations
        10. 2.3.5.10 Vaguer moments in time
      6. 2.3.6 Legacy colors
      7. 2.3.7 Space-separated tokens
      8. 2.3.8 Comma-separated tokens
      9. 2.3.9 References
      10. 2.3.10 Media queries
      11. 2.3.11 Unique internal values
    4. 2.4 URLs
      1. 2.4.1 Terminology
      2. 2.4.2 Parsing URLs
      3. 2.4.3 Dynamic changes to base URLs
    5. 2.5 Fetching resources
      1. 2.5.1 Terminology
      2. 2.5.2 Determining the type of a resource
      3. 2.5.3 Extracting character encodings from meta elements
      4. 2.5.4 CORS settings attributes
      5. 2.5.5 Referrer policy attributes
      6. 2.5.6 Nonce attributes
      7. 2.5.7 Lazy loading attributes
      8. 2.5.8 Blocking attributes
      9. 2.5.9 Fetch priority attributes
    6. 2.6 Common DOM interfaces
      1. 2.6.1 Reflecting content attributes in IDL attributes
      2. 2.6.2 Using reflect in specifications
      3. 2.6.3 Collections
        1. 2.6.3.1 The HTMLAllCollection interface
          1. 2.6.3.1.1 [[Call]] ( thisArgument, argumentsList )
        2. 2.6.3.2 The HTMLFormControlsCollection interface
        3. 2.6.3.3 The HTMLOptionsCollection interface
      4. 2.6.4 The DOMStringList interface
    7. 2.7 Safe passing of structured data
      1. 2.7.1 Serializable objects
      2. 2.7.2 Transferable objects
      3. 2.7.3 StructuredSerializeInternal ( value, forStorage [ , memory ] )
      4. 2.7.4 StructuredSerialize ( value )
      5. 2.7.5 StructuredSerializeForStorage ( value )
      6. 2.7.6 StructuredDeserialize ( serialized, targetRealm [ , memory ] )
      7. 2.7.7 StructuredSerializeWithTransfer ( value, transferList )
      8. 2.7.8 StructuredDeserializeWithTransfer ( serializeWithTransferResult, targetRealm )
      9. 2.7.9 Performing serialization and transferring from other specifications
      10. 2.7.10 Structured cloning API
  3. 3 Semantics, structure, and APIs of HTML documents
    1. 3.1 Documents
      1. 3.1.1 The Document object
      2. 3.1.2 The DocumentOrShadowRoot interface
      3. 3.1.3 Resource metadata management
      4. 3.1.4 Reporting document loading status
      5. 3.1.5 Render-blocking mechanism
      6. 3.1.6 DOM tree accessors
    2. 3.2 Elements
      1. 3.2.1 Semantics
      2. 3.2.2 Elements in the DOM
      3. 3.2.3 HTML element constructors
      4. 3.2.4 Element definitions
        1. 3.2.4.1 Attributes
      5. 3.2.5 Content models
        1. 3.2.5.1 The "nothing" content model
        2. 3.2.5.2 Kinds of content
          1. 3.2.5.2.1 Metadata content
          2. 3.2.5.2.2 Flow content
          3. 3.2.5.2.3 Sectioning content
          4. 3.2.5.2.4 Heading content
          5. 3.2.5.2.5 Phrasing content
          6. 3.2.5.2.6 Embedded content
          7. 3.2.5.2.7 Interactive content
          8. 3.2.5.2.8 Palpable content
          9. 3.2.5.2.9 Script-supporting elements
        3. 3.2.5.3 Transparent content models
        4. 3.2.5.4 Paragraphs
      6. 3.2.6 Global attributes
        1. 3.2.6.1 The title attribute
        2. 3.2.6.2 The lang and xml:lang attributes
        3. 3.2.6.3 The translate attribute
        4. 3.2.6.4 The dir attribute
        5. 3.2.6.5 The style attribute
        6. 3.2.6.6 Embedding custom non-visible data with the data-* attributes
      7. 3.2.7 The innerText and outerText properties
      8. 3.2.8 Requirements relating to the bidirectional algorithm
        1. 3.2.8.1 Authoring conformance criteria for bidirectional-algorithm formatting characters
        2. 3.2.8.2 User agent conformance criteria
      9. 3.2.9 Requirements related to ARIA and to platform accessibility APIs
  4. 4 The elements of HTML
    1. 4.1 The document element
      1. 4.1.1 The html element
    2. 4.2 Document metadata
      1. 4.2.1 The head element
      2. 4.2.2 The title element
      3. 4.2.3 The base element
      4. 4.2.4 The link element
        1. 4.2.4.1 Processing the media attribute
        2. 4.2.4.2 Processing the type attribute
        3. 4.2.4.3 Fetching and processing a resource from a link element
        4. 4.2.4.4 Processing `Link` headers
        5. 4.2.4.5 Early hints
        6. 4.2.4.6 Providing users with a means to follow hyperlinks created using the link element
      5. 4.2.5 The meta element
        1. 4.2.5.1 Standard metadata names
        2. 4.2.5.2 Other metadata names
        3. 4.2.5.3 Pragma directives
        4. 4.2.5.4 Specifying the document's character encoding
      6. 4.2.6 The style element
      7. 4.2.7 Interactions of styling and scripting
    3. 4.3 Sections
      1. 4.3.1 The body element
      2. 4.3.2 The article element
      3. 4.3.3 The section element
      4. 4.3.4 The nav element
      5. 4.3.5 The aside element
      6. 4.3.6 The h1, h2, h3, h4, h5, and h6 elements
      7. 4.3.7 The hgroup element
      8. 4.3.8 The header element
      9. 4.3.9 The footer element
      10. 4.3.10 The address element
      11. 4.3.11 Headings and outlines
        1. 4.3.11.1 Sample outlines
        2. 4.3.11.2 Exposing outlines to users
      12. 4.3.12 Usage summary
        1. 4.3.12.1 Article or section?
    4. 4.4 Grouping content
      1. 4.4.1 The p element
      2. 4.4.2 The hr element
      3. 4.4.3 The pre element
      4. 4.4.4 The blockquote element
      5. 4.4.5 The ol element
      6. 4.4.6 The ul element
      7. 4.4.7 The menu element
      8. 4.4.8 The li element
      9. 4.4.9 The dl element
      10. 4.4.10 The dt element
      11. 4.4.11 The dd element
      12. 4.4.12 The figure element
      13. 4.4.13 The figcaption element
      14. 4.4.14 The main element
      15. 4.4.15 The search element
      16. 4.4.16 The div element
    5. 4.5 Text-level semantics
      1. 4.5.1 The a element
      2. 4.5.2 The em element
      3. 4.5.3 The strong element
      4. 4.5.4 The small element
      5. 4.5.5 The s element
      6. 4.5.6 The cite element
      7. 4.5.7 The q element
      8. 4.5.8 The dfn element
      9. 4.5.9 The abbr element
      10. 4.5.10 The ruby element
      11. 4.5.11 The rt element
      12. 4.5.12 The rp element
      13. 4.5.13 The data element
      14. 4.5.14 The time element
      15. 4.5.15 The code element
      16. 4.5.16 The var element
      17. 4.5.17 The samp element
      18. 4.5.18 The kbd element
      19. 4.5.19 The sub and sup elements
      20. 4.5.20 The i element
      21. 4.5.21 The b element
      22. 4.5.22 The u element
      23. 4.5.23 The mark element
      24. 4.5.24 The bdi element
      25. 4.5.25 The bdo element
      26. 4.5.26 The span element
      27. 4.5.27 The br element
      28. 4.5.28 The wbr element
      29. 4.5.29 Usage summary
    6. 4.6 Links
      1. 4.6.1 Introduction
      2. 4.6.2 Links created by a and area elements
      3. 4.6.3 API for a and area elements
      4. 4.6.4 Following hyperlinks
      5. 4.6.5 Downloading resources
      6. 4.6.6 Hyperlink auditing
        1. 4.6.6.1 The `Ping-From` and `Ping-To` headers
      7. 4.6.7 Link types
        1. 4.6.7.1 Link type "alternate"
        2. 4.6.7.2 Link type "author"
        3. 4.6.7.3 Link type "bookmark"
        4. 4.6.7.4 Link type "canonical"
        5. 4.6.7.5 Link type "dns-prefetch"
        6. 4.6.7.6 Link type "expect"
        7. 4.6.7.7 Link type "external"
        8. 4.6.7.8 Link type "help"
        9. 4.6.7.9 Link type "icon"
        10. 4.6.7.10 Link type "license"
        11. 4.6.7.11 Link type "manifest"
        12. 4.6.7.12 Link type "modulepreload"
        13. 4.6.7.13 Link type "nofollow"
        14. 4.6.7.14 Link type "noopener"
        15. 4.6.7.15 Link type "noreferrer"
        16. 4.6.7.16 Link type "opener"
        17. 4.6.7.17 Link type "pingback"
        18. 4.6.7.18 Link type "preconnect"
        19. 4.6.7.19 Link type "prefetch"
        20. 4.6.7.20 Link type "preload"
        21. 4.6.7.21 Link type "privacy-policy"
        22. 4.6.7.22 Link type "search"
        23. 4.6.7.23 Link type "stylesheet"
        24. 4.6.7.24 Link type "tag"
        25. 4.6.7.25 Link Type "terms-of-service"
        26. 4.6.7.26 Sequential link types
          1. 4.6.7.26.1 Link type "next"
          2. 4.6.7.26.2 Link type "prev"
        27. 4.6.7.27 Other link types
    7. 4.7 Edits
      1. 4.7.1 The ins element
      2. 4.7.2 The del element
      3. 4.7.3 Attributes common to ins and del elements
      4. 4.7.4 Edits and paragraphs
      5. 4.7.5 Edits and lists
      6. 4.7.6 Edits and tables
    8. 4.8 Embedded content
      1. 4.8.1 The picture element
      2. 4.8.2 The source element
      3. 4.8.3 The img element
      4. 4.8.4 Images
        1. 4.8.4.1 Introduction
          1. 4.8.4.1.1 Adaptive images
        2. 4.8.4.2 Attributes common to source, img, and link elements
          1. 4.8.4.2.1 Srcset attributes
          2. 4.8.4.2.2 Sizes attributes
        3. 4.8.4.3 Processing model
          1. 4.8.4.3.1 When to obtain images
          2. 4.8.4.3.2 Reacting to DOM mutations
          3. 4.8.4.3.3 The list of available images
          4. 4.8.4.3.4 Decoding images
          5. 4.8.4.3.5 Updating the image data
          6. 4.8.4.3.6 Preparing an image for presentation
          7. 4.8.4.3.7 Selecting an image source
          8. 4.8.4.3.8 Creating a source set from attributes
          9. 4.8.4.3.9 Updating the source set
          10. 4.8.4.3.10 Parsing a srcset attribute
          11. 4.8.4.3.11 Parsing a sizes attribute
          12. 4.8.4.3.12 Normalizing the source densities
          13. 4.8.4.3.13 Reacting to environment changes
        4. 4.8.4.4 Requirements for providing text to act as an alternative for images
          1. 4.8.4.4.1 General guidelines
          2. 4.8.4.4.2 A link or button containing nothing but the image
          3. 4.8.4.4.3 A phrase or paragraph with an alternative graphical representation: charts, diagrams, graphs, maps, illustrations
          4. 4.8.4.4.4 A short phrase or label with an alternative graphical representation: icons, logos
          5. 4.8.4.4.5 Text that has been rendered to a graphic for typographical effect
          6. 4.8.4.4.6 A graphical representation of some of the surrounding text
          7. 4.8.4.4.7 Ancillary images
          8. 4.8.4.4.8 A purely decorative image that doesn't add any information
          9. 4.8.4.4.9 A group of images that form a single larger picture with no links
          10. 4.8.4.4.10 A group of images that form a single larger picture with links
          11. 4.8.4.4.11 A key part of the content
          12. 4.8.4.4.12 An image not intended for the user
          13. 4.8.4.4.13 An image in an email or private document intended for a specific person who is known to be able to view images
          14. 4.8.4.4.14 Guidance for markup generators
          15. 4.8.4.4.15 Guidance for conformance checkers
      5. 4.8.5 The iframe element
      6. 4.8.6 The embed element
      7. 4.8.7 The object element
      8. 4.8.8 The video element
      9. 4.8.9 The audio element
      10. 4.8.10 The track element
      11. 4.8.11 Media elements
        1. 4.8.11.1 Error codes
        2. 4.8.11.2 Location of the media resource
        3. 4.8.11.3 MIME types
        4. 4.8.11.4 Network states
        5. 4.8.11.5 Loading the media resource
        6. 4.8.11.6 Offsets into the media resource
        7. 4.8.11.7 Ready states
        8. 4.8.11.8 Playing the media resource
        9. 4.8.11.9 Seeking
        10. 4.8.11.10 Media resources with multiple media tracks
          1. 4.8.11.10.1 AudioTrackList and VideoTrackList objects
          2. 4.8.11.10.2 Selecting specific audio and video tracks declaratively
        11. 4.8.11.11 Timed text tracks
          1. 4.8.11.11.1 Text track model
          2. 4.8.11.11.2 Sourcing in-band text tracks
          3. 4.8.11.11.3 Sourcing out-of-band text tracks
          4. 4.8.11.11.4 Guidelines for exposing cues in various formats as text track cues
          5. 4.8.11.11.5 Text track API
          6. 4.8.11.11.6 Event handlers for objects of the text track APIs
          7. 4.8.11.11.7 Best practices for metadata text tracks
        12. 4.8.11.12 Identifying a track kind through a URL
        13. 4.8.11.13 User interface
        14. 4.8.11.14 Time ranges
        15. 4.8.11.15 The TrackEvent interface
        16. 4.8.11.16 Events summary
        17. 4.8.11.17 Security and privacy considerations
        18. 4.8.11.18 Best practices for authors using media elements
        19. 4.8.11.19 Best practices for implementers of media elements
      12. 4.8.12 The map element
      13. 4.8.13 The area element
      14. 4.8.14 Image maps
        1. 4.8.14.1 Authoring
        2. 4.8.14.2 Processing model
      15. 4.8.15 MathML
      16. 4.8.16 SVG
      17. 4.8.17 Dimension attributes
    9. 4.9 Tabular data
      1. 4.9.1 The table element
        1. 4.9.1.1 Techniques for describing tables
        2. 4.9.1.2 Techniques for table design
      2. 4.9.2 The caption element
      3. 4.9.3 The colgroup element
      4. 4.9.4 The col element
      5. 4.9.5 The tbody element
      6. 4.9.6 The thead element
      7. 4.9.7 The tfoot element
      8. 4.9.8 The tr element
      9. 4.9.9 The td element
      10. 4.9.10 The th element
      11. 4.9.11 Attributes common to td and th elements
      12. 4.9.12 Processing model
        1. 4.9.12.1 Forming a table
        2. 4.9.12.2 Forming relationships between data cells and header cells
      13. 4.9.13 Examples
    10. 4.10 Forms
      1. 4.10.1 Introduction
        1. 4.10.1.1 Writing a form's user interface
        2. 4.10.1.2 Implementing the server-side processing for a form
        3. 4.10.1.3 Configuring a form to communicate with a server
        4. 4.10.1.4 Client-side form validation
        5. 4.10.1.5 Enabling client-side automatic filling of form controls
        6. 4.10.1.6 Improving the user experience on mobile devices
        7. 4.10.1.7 The difference between the field type, the autofill field name, and the input modality
        8. 4.10.1.8 Date, time, and number formats
      2. 4.10.2 Categories
      3. 4.10.3 The form element
      4. 4.10.4 The label element
      5. 4.10.5 The input element
        1. 4.10.5.1 States of the type attribute
          1. 4.10.5.1.1 Hidden state (type=hidden)
          2. 4.10.5.1.2 Text (type=text) state and Search state (type=search)
          3. 4.10.5.1.3 Telephone state (type=tel)
          4. 4.10.5.1.4 URL state (type=url)
          5. 4.10.5.1.5 Email state (type=email)
          6. 4.10.5.1.6 Password state (type=password)
          7. 4.10.5.1.7 Date state (type=date)
          8. 4.10.5.1.8 Month state (type=month)
          9. 4.10.5.1.9 Week state (type=week)
          10. 4.10.5.1.10 Time state (type=time)
          11. 4.10.5.1.11 Local Date and Time state (type=datetime-local)
          12. 4.10.5.1.12 Number state (type=number)
          13. 4.10.5.1.13 Range state (type=range)
          14. 4.10.5.1.14 Color state (type=color)
          15. 4.10.5.1.15 Checkbox state (type=checkbox)
          16. 4.10.5.1.16 Radio Button state (type=radio)
          17. 4.10.5.1.17 File Upload state (type=file)
          18. 4.10.5.1.18 Submit Button state (type=submit)
          19. 4.10.5.1.19 Image Button state (type=image)
          20. 4.10.5.1.20 Reset Button state (type=reset)
          21. 4.10.5.1.21 Button state (type=button)
        2. 4.10.5.2 Implementation notes regarding localization of form controls
        3. 4.10.5.3 Common input element attributes
          1. 4.10.5.3.1 The maxlength and minlength attributes
          2. 4.10.5.3.2 The size attribute
          3. 4.10.5.3.3 The readonly attribute
          4. 4.10.5.3.4 The required attribute
          5. 4.10.5.3.5 The multiple attribute
          6. 4.10.5.3.6 The pattern attribute
          7. 4.10.5.3.7 The min and max attributes
          8. 4.10.5.3.8 The step attribute
          9. 4.10.5.3.9 The list attribute
          10. 4.10.5.3.10 The placeholder attribute
        4. 4.10.5.4 Common input element APIs
        5. 4.10.5.5 Common event behaviors
      6. 4.10.6 The button element
      7. 4.10.7 The select element
      8. 4.10.8 The datalist element
      9. 4.10.9 The optgroup element
      10. 4.10.10 The option element
      11. 4.10.11 The textarea element
      12. 4.10.12 The output element
      13. 4.10.13 The progress element
      14. 4.10.14 The meter element
      15. 4.10.15 The fieldset element
      16. 4.10.16 The legend element
      17. 4.10.17 Form control infrastructure
        1. 4.10.17.1 A form control's value
        2. 4.10.17.2 Mutability
        3. 4.10.17.3 Association of controls and forms
      18. 4.10.18 Attributes common to form controls
        1. 4.10.18.1 Naming form controls: the name attribute
        2. 4.10.18.2 Submitting element directionality: the dirname attribute
        3. 4.10.18.3 Limiting user input length: the maxlength attribute
        4. 4.10.18.4 Setting minimum input length requirements: the minlength attribute
        5. 4.10.18.5 Enabling and disabling form controls: the disabled attribute
        6. 4.10.18.6 Form submission attributes
        7. 4.10.18.7 Autofill
          1. 4.10.18.7.1 Autofilling form controls: the autocomplete attribute
          2. 4.10.18.7.2 Processing model
      19. 4.10.19 APIs for the text control selections
      20. 4.10.20 Constraints
        1. 4.10.20.1 Definitions
        2. 4.10.20.2 Constraint validation
        3. 4.10.20.3 The constraint validation API
        4. 4.10.20.4 Security
      21. 4.10.21 Form submission
        1. 4.10.21.1 Introduction
        2. 4.10.21.2 Implicit submission
        3. 4.10.21.3 Form submission algorithm
        4. 4.10.21.4 Constructing the entry list
        5. 4.10.21.5 Selecting a form submission encoding
        6. 4.10.21.6 Converting an entry list to a list of name-value pairs
        7. 4.10.21.7 URL-encoded form data
        8. 4.10.21.8 Multipart form data
        9. 4.10.21.9 Plain text form data
        10. 4.10.21.10 The SubmitEvent interface
        11. 4.10.21.11 The FormDataEvent interface
      22. 4.10.22 Resetting a form
    11. 4.11 Interactive elements
      1. 4.11.1 The details element
      2. 4.11.2 The summary element
      3. 4.11.3 Commands
        1. 4.11.3.1 Facets
        2. 4.11.3.2 Using the a element to define a command
        3. 4.11.3.3 Using the button element to define a command
        4. 4.11.3.4 Using the input element to define a command
        5. 4.11.3.5 Using the option element to define a command
        6. 4.11.3.6 Using the accesskey attribute on a legend element to define a command
        7. 4.11.3.7 Using the accesskey attribute to define a command on other elements
      4. 4.11.4 The dialog element
    12. 4.12 Scripting
      1. 4.12.1 The script element
        1. 4.12.1.1 Processing model
        2. 4.12.1.2 Scripting languages
        3. 4.12.1.3 Restrictions for contents of script elements
        4. 4.12.1.4 Inline documentation for external scripts
        5. 4.12.1.5 Interaction of script elements and XSLT
      2. 4.12.2 The noscript element
      3. 4.12.3 The template element
        1. 4.12.3.1 Interaction of template elements with XSLT and XPath
      4. 4.12.4 The slot element
      5. 4.12.5 The canvas element
        1. 4.12.5.1 The 2D rendering context
          1. 4.12.5.1.1 Implementation notes
          2. 4.12.5.1.2 The canvas state
          3. 4.12.5.1.3 Line styles
          4. 4.12.5.1.4 Text styles
          5. 4.12.5.1.5 Building paths
          6. 4.12.5.1.6 Path2D objects
          7. 4.12.5.1.7 Transformations
          8. 4.12.5.1.8 Image sources for 2D rendering contexts
          9. 4.12.5.1.9 Fill and stroke styles
          10. 4.12.5.1.10 Drawing rectangles to the bitmap
          11. 4.12.5.1.11 Drawing text to the bitmap
          12. 4.12.5.1.12 Drawing paths to the canvas
          13. 4.12.5.1.13 Drawing focus rings
          14. 4.12.5.1.14 Drawing images
          15. 4.12.5.1.15 Pixel manipulation
          16. 4.12.5.1.16 Compositing
          17. 4.12.5.1.17 Image smoothing
          18. 4.12.5.1.18 Shadows
          19. 4.12.5.1.19 Filters
          20. 4.12.5.1.20 Working with externally-defined SVG filters
          21. 4.12.5.1.21 Drawing model
          22. 4.12.5.1.22 Best practices
          23. 4.12.5.1.23 Examples
        2. 4.12.5.2 The ImageBitmap rendering context
          1. 4.12.5.2.1 Introduction
          2. 4.12.5.2.2 The ImageBitmapRenderingContext interface
        3. 4.12.5.3 The OffscreenCanvas interface
          1. 4.12.5.3.1 The offscreen 2D rendering context
        4. 4.12.5.4 Color spaces and color space conversion
        5. 4.12.5.5 Serializing bitmaps to a file
        6. 4.12.5.6 Security with canvas elements
        7. 4.12.5.7 Premultiplied alpha and the 2D rendering context
    13. 4.13 Custom elements
      1. 4.13.1 Introduction
        1. 4.13.1.1 Creating an autonomous custom element
        2. 4.13.1.2 Creating a form-associated custom element
        3. 4.13.1.3 Creating a custom element with default accessible roles, states, and properties
        4. 4.13.1.4 Creating a customized built-in element
        5. 4.13.1.5 Drawbacks of autonomous custom elements
        6. 4.13.1.6 Upgrading elements after their creation
        7. 4.13.1.7 Exposing custom element states
      2. 4.13.2 Requirements for custom element constructors and reactions
      3. 4.13.3 Core concepts
      4. 4.13.4 The CustomElementRegistry interface
      5. 4.13.5 Upgrades
      6. 4.13.6 Custom element reactions
      7. 4.13.7 Element internals
        1. 4.13.7.1 The ElementInternals interface
        2. 4.13.7.2 Shadow root access
        3. 4.13.7.3 Form-associated custom elements
        4. 4.13.7.4 Accessibility semantics
        5. 4.13.7.5 Custom state pseudo-class
    14. 4.14 Common idioms without dedicated elements
      1. 4.14.1 Breadcrumb navigation
      2. 4.14.2 Tag clouds
      3. 4.14.3 Conversations
      4. 4.14.4 Footnotes
    15. 4.15 Disabled elements
    16. 4.16 Matching HTML elements using selectors and CSS
      1. 4.16.1 Case-sensitivity of the CSS 'attr()' function
      2. 4.16.2 Case-sensitivity of selectors
      3. 4.16.3 Pseudo-classes
  5. 5 Microdata
    1. 5.1 Introduction
      1. 5.1.1 Overview
      2. 5.1.2 The basic syntax
      3. 5.1.3 Typed items
      4. 5.1.4 Global identifiers for items
      5. 5.1.5 Selecting names when defining vocabularies
    2. 5.2 Encoding microdata
      1. 5.2.1 The microdata model
      2. 5.2.2 Items
      3. 5.2.3 Names: the itemprop attribute
      4. 5.2.4 Values
      5. 5.2.5 Associating names with items
      6. 5.2.6 Microdata and other namespaces
    3. 5.3 Sample microdata vocabularies
      1. 5.3.1 vCard
        1. 5.3.1.1 Conversion to vCard
        2. 5.3.1.2 Examples
      2. 5.3.2 vEvent
        1. 5.3.2.1 Conversion to iCalendar
        2. 5.3.2.2 Examples
      3. 5.3.3 Licensing works
        1. 5.3.3.1 Examples
    4. 5.4 Converting HTML to other formats
      1. 5.4.1 JSON
  6. 6 User interaction
    1. 6.1 The hidden attribute
    2. 6.2 Page visibility
      1. 6.2.1 The VisibilityStateEntry interface
    3. 6.3 Inert subtrees
      1. 6.3.1 Modal dialogs and inert subtrees
      2. 6.3.2 The inert attribute
    4. 6.4 Tracking user activation
      1. 6.4.1 Data model
      2. 6.4.2 Processing model
      3. 6.4.3 APIs gated by user activation
      4. 6.4.4 The UserActivation interface
      5. 6.4.5 User agent automation
    5. 6.5 Activation behavior of elements
      1. 6.5.1 The ToggleEvent interface
    6. 6.6 Focus
      1. 6.6.1 Introduction
      2. 6.6.2 Data model
      3. 6.6.3 The tabindex attribute
      4. 6.6.4 Processing model
      5. 6.6.5 Sequential focus navigation
      6. 6.6.6 Focus management APIs
      7. 6.6.7 The autofocus attribute
    7. 6.7 Assigning keyboard shortcuts
      1. 6.7.1 Introduction
      2. 6.7.2 The accesskey attribute
      3. 6.7.3 Processing model
    8. 6.8 Editing
      1. 6.8.1 Making document regions editable: The contenteditable content attribute
      2. 6.8.2 Making entire documents editable: the designMode getter and setter
      3. 6.8.3 Best practices for in-page editors
      4. 6.8.4 Editing APIs
      5. 6.8.5 Spelling and grammar checking
      6. 6.8.6 Writing suggestions
      7. 6.8.7 Autocapitalization
      8. 6.8.8 Autocorrection
      9. 6.8.9 Input modalities: the inputmode attribute
      10. 6.8.10 Input modalities: the enterkeyhint attribute
    9. 6.9 Find-in-page
      1. 6.9.1 Introduction
      2. 6.9.2 Interaction with details and hidden=until-found
      3. 6.9.3 Interaction with selection
    10. 6.10 Close requests and close watchers
      1. 6.10.1 Close requests
      2. 6.10.2 Close watcher infrastructure
      3. 6.10.3 The CloseWatcher interface
    11. 6.11 Drag and drop
      1. 6.11.1 Introduction
      2. 6.11.2 The drag data store
      3. 6.11.3 The DataTransfer interface
        1. 6.11.3.1 The DataTransferItemList interface
        2. 6.11.3.2 The DataTransferItem interface
      4. 6.11.4 The DragEvent interface
      5. 6.11.5 Processing model
      6. 6.11.6 Events summary
      7. 6.11.7 The draggable attribute
      8. 6.11.8 Security risks in the drag-and-drop model
    12. 6.12 The popover attribute
      1. 6.12.1 The popover target attributes
      2. 6.12.2 Popover light dismiss
  7. 7 Loading web pages
    1. 7.1 Supporting concepts
      1. 7.1.1 Origins
        1. 7.1.1.1 Sites
        2. 7.1.1.2 Relaxing the same-origin restriction
      2. 7.1.2 Origin-keyed agent clusters
      3. 7.1.3 Cross-origin opener policies
        1. 7.1.3.1 The headers
        2. 7.1.3.2 Browsing context group switches due to opener policy
        3. 7.1.3.3 Reporting
      4. 7.1.4 Cross-origin embedder policies
        1. 7.1.4.1 The headers
        2. 7.1.4.2 Embedder policy checks
      5. 7.1.5 Sandboxing
      6. 7.1.6 Policy containers
    2. 7.2 APIs related to navigation and session history
      1. 7.2.1 Security infrastructure for Window, WindowProxy, and Location objects
        1. 7.2.1.1 Integration with IDL
        2. 7.2.1.2 Shared internal slot: [[CrossOriginPropertyDescriptorMap]]
        3. 7.2.1.3 Shared abstract operations
          1. 7.2.1.3.1 CrossOriginProperties ( O )
          2. 7.2.1.3.2 CrossOriginPropertyFallback ( P )
          3. 7.2.1.3.3 IsPlatformObjectSameOrigin ( O )
          4. 7.2.1.3.4 CrossOriginGetOwnPropertyHelper ( O, P )
          5. 7.2.1.3.5 CrossOriginGet ( O, P, Receiver )
          6. 7.2.1.3.6 CrossOriginSet ( O, P, V, Receiver )
          7. 7.2.1.3.7 CrossOriginOwnPropertyKeys ( O )
      2. 7.2.2 The Window object
        1. 7.2.2.1 Opening and closing windows
        2. 7.2.2.2 Indexed access on the Window object
        3. 7.2.2.3 Named access on the Window object
        4. 7.2.2.4 Accessing related windows
        5. 7.2.2.5 Historical browser interface element APIs
        6. 7.2.2.6 Script settings for Window objects
      3. 7.2.3 The WindowProxy exotic object
        1. 7.2.3.1 [[GetPrototypeOf]] ( )
        2. 7.2.3.2 [[SetPrototypeOf]] ( V )
        3. 7.2.3.3 [[IsExtensible]] ( )
        4. 7.2.3.4 [[PreventExtensions]] ( )
        5. 7.2.3.5 [[GetOwnProperty]] ( P )
        6. 7.2.3.6 [[DefineOwnProperty]] ( P, Desc )
        7. 7.2.3.7 [[Get]] ( P, Receiver )
        8. 7.2.3.8 [[Set]] ( P, V, Receiver )
        9. 7.2.3.9 [[Delete]] ( P )
        10. 7.2.3.10 [[OwnPropertyKeys]] ( )
      4. 7.2.4 The Location interface
        1. 7.2.4.1 [[GetPrototypeOf]] ( )
        2. 7.2.4.2 [[SetPrototypeOf]] ( V )
        3. 7.2.4.3 [[IsExtensible]] ( )
        4. 7.2.4.4 [[PreventExtensions]] ( )
        5. 7.2.4.5 [[GetOwnProperty]] ( P )
        6. 7.2.4.6 [[DefineOwnProperty]] ( P, Desc )
        7. 7.2.4.7 [[Get]] ( P, Receiver )
        8. 7.2.4.8 [[Set]] ( P, V, Receiver )
        9. 7.2.4.9 [[Delete]] ( P )
        10. 7.2.4.10 [[OwnPropertyKeys]] ( )
      5. 7.2.5 The History interface
      6. 7.2.6 The navigation API
        1. 7.2.6.1 Introduction
        2. 7.2.6.2 The Navigation interface
        3. 7.2.6.3 Core infrastructure
        4. 7.2.6.4 Initializing and updating the entry list
        5. 7.2.6.5 The NavigationHistoryEntry interface
        6. 7.2.6.6 The history entry list
        7. 7.2.6.7 Initiating navigations
        8. 7.2.6.8 Ongoing navigation tracking
        9. 7.2.6.9 The NavigationActivation interface
        10. 7.2.6.10 The navigate event
          1. 7.2.6.10.1 The NavigateEvent interface
          2. 7.2.6.10.2 The NavigationDestination interface
          3. 7.2.6.10.3 Firing the event
          4. 7.2.6.10.4 Scroll and focus behavior
      7. 7.2.7 Event interfaces
        1. 7.2.7.1 The NavigationCurrentEntryChangeEvent interface
        2. 7.2.7.2 The PopStateEvent interface
        3. 7.2.7.3 The HashChangeEvent interface
        4. 7.2.7.4 The PageSwapEvent interface
        5. 7.2.7.5 The PageRevealEvent interface
        6. 7.2.7.6 The PageTransitionEvent interface
        7. 7.2.7.7 The BeforeUnloadEvent interface
      8. 7.2.8 The NotRestoredReasons interface
    3. 7.3 Infrastructure for sequences of documents
      1. 7.3.1 Navigables
        1. 7.3.1.1 Traversable navigables
        2. 7.3.1.2 Top-level traversables
        3. 7.3.1.3 Child navigables
        4. 7.3.1.4 Jake diagrams
        5. 7.3.1.5 Related navigable collections
        6. 7.3.1.6 Navigable destruction
        7. 7.3.1.7 Navigable target names
      2. 7.3.2 Browsing contexts
        1. 7.3.2.1 Creating browsing contexts
        2. 7.3.2.2 Related browsing contexts
        3. 7.3.2.3 Groupings of browsing contexts
      3. 7.3.3 Fully active documents
    4. 7.4 Navigation and session history
      1. 7.4.1 Session history
        1. 7.4.1.1 Session history entries
        2. 7.4.1.2 Document state
        3. 7.4.1.3 Centralized modifications of session history
        4. 7.4.1.4 Low-level operations on session history
      2. 7.4.2 Navigation
        1. 7.4.2.1 Supporting concepts
        2. 7.4.2.2 Beginning navigation
        3. 7.4.2.3 Ending navigation
          1. 7.4.2.3.1 The usual cross-document navigation case
          2. 7.4.2.3.2 The javascript: URL special case
          3. 7.4.2.3.3 Fragment navigations
          4. 7.4.2.3.4 Non-fetch schemes and external software
        4. 7.4.2.4 Preventing navigation
        5. 7.4.2.5 Aborting navigation
      3. 7.4.3 Reloading and traversing
      4. 7.4.4 Non-fragment synchronous "navigations"
      5. 7.4.5 Populating a session history entry
      6. 7.4.6 Applying the history step
        1. 7.4.6.1 Updating the traversable
        2. 7.4.6.2 Updating the document
        3. 7.4.6.3 Revealing the document
        4. 7.4.6.4 Scrolling to a fragment
        5. 7.4.6.5 Persisted history entry state
    5. 7.5 Document lifecycle
      1. 7.5.1 Shared document creation infrastructure
      2. 7.5.2 Loading HTML documents
      3. 7.5.3 Loading XML documents
      4. 7.5.4 Loading text documents
      5. 7.5.5 Loading multipart/x-mixed-replace documents
      6. 7.5.6 Loading media documents
      7. 7.5.7 Loading a document for inline content that doesn't have a DOM
      8. 7.5.8 Finishing the loading process
      9. 7.5.9 Unloading documents
      10. 7.5.10 Destroying documents
      11. 7.5.11 Aborting a document load
    6. 7.6 The `X-Frame-Options` header
    7. 7.7 The `Refresh` header
    8. 7.8 Browser user interface considerations
  8. 8 Web application APIs
    1. 8.1 Scripting
      1. 8.1.1 Introduction
      2. 8.1.2 Agents and agent clusters
        1. 8.1.2.1 Integration with the JavaScript agent formalism
        2. 8.1.2.2 Integration with the JavaScript agent cluster formalism
      3. 8.1.3 Realms and their counterparts
        1. 8.1.3.1 Environments
        2. 8.1.3.2 Environment settings objects
        3. 8.1.3.3 Realms, settings objects, and global objects
          1. 8.1.3.3.1 Entry
          2. 8.1.3.3.2 Incumbent
          3. 8.1.3.3.3 Current
          4. 8.1.3.3.4 Relevant
        4. 8.1.3.4 Enabling and disabling scripting
        5. 8.1.3.5 Secure contexts
      4. 8.1.4 Script processing model
        1. 8.1.4.1 Scripts
        2. 8.1.4.2 Fetching scripts
        3. 8.1.4.3 Creating scripts
        4. 8.1.4.4 Calling scripts
        5. 8.1.4.5 Killing scripts
        6. 8.1.4.6 Runtime script errors
        7. 8.1.4.7 Unhandled promise rejections
        8. 8.1.4.8 Import map parse results
      5. 8.1.5 Module specifier resolution
        1. 8.1.5.1 The resolution algorithm
        2. 8.1.5.2 Import maps
        3. 8.1.5.3 Import map processing model
      6. 8.1.6 JavaScript specification host hooks
        1. 8.1.6.1 HostEnsureCanAddPrivateElement(O)
        2. 8.1.6.2 HostEnsureCanCompileStrings(realm, parameterStrings, bodyString, codeString, compilationType, parameterArgs, bodyArg)
        3. 8.1.6.3 HostGetCodeForEval(argument)
        4. 8.1.6.4 HostPromiseRejectionTracker(promise, operation)
        5. 8.1.6.5 HostSystemUTCEpochNanoseconds(global)
        6. 8.1.6.6 Job-related host hooks
          1. 8.1.6.6.1 HostCallJobCallback(callback, V, argumentsList)
          2. 8.1.6.6.2 HostEnqueueFinalizationRegistryCleanupJob(finalizationRegistry)
          3. 8.1.6.6.3 HostEnqueueGenericJob(job, realm)
          4. 8.1.6.6.4 HostEnqueuePromiseJob(job, realm)
          5. 8.1.6.6.5 HostEnqueueTimeoutJob(job, realm, milliseconds)
          6. 8.1.6.6.6 HostMakeJobCallback(callable)
        7. 8.1.6.7 Module-related host hooks
          1. 8.1.6.7.1 HostGetImportMetaProperties(moduleRecord)
          2. 8.1.6.7.2 HostGetSupportedImportAttributes()
          3. 8.1.6.7.3 HostLoadImportedModule(referrer, moduleRequest, loadState, payload)
      7. 8.1.7 Event loops
        1. 8.1.7.1 Definitions
        2. 8.1.7.2 Queuing tasks
        3. 8.1.7.3 Processing model
        4. 8.1.7.4 Generic task sources
        5. 8.1.7.5 Dealing with the event loop from other specifications
      8. 8.1.8 Events
        1. 8.1.8.1 Event handlers
        2. 8.1.8.2 Event handlers on elements, Document objects, and Window objects
          1. 8.1.8.2.1 IDL definitions
        3. 8.1.8.3 Event firing
    2. 8.2 The WindowOrWorkerGlobalScope mixin
    3. 8.3 Base64 utility methods
    4. 8.4 Dynamic markup insertion
      1. 8.4.1 Opening the input stream
      2. 8.4.2 Closing the input stream
      3. 8.4.3 document.write()
      4. 8.4.4 document.writeln()
    5. 8.5 DOM parsing and serialization APIs
      1. 8.5.1 The DOMParser interface
      2. 8.5.2 Unsafe HTML parsing methods
      3. 8.5.3 HTML serialization methods
      4. 8.5.4 The innerHTML property
      5. 8.5.5 The outerHTML property
      6. 8.5.6 The insertAdjacentHTML() method
      7. 8.5.7 The createContextualFragment() method
    6. 8.6 Timers
    7. 8.7 Microtask queuing
    8. 8.8 User prompts
      1. 8.8.1 Simple dialogs
      2. 8.8.2 Printing
    9. 8.9 System state and capabilities
      1. 8.9.1 The Navigator object
        1. 8.9.1.1 Client identification
        2. 8.9.1.2 Language preferences
        3. 8.9.1.3 Browser state
        4. 8.9.1.4 Custom scheme handlers: the registerProtocolHandler() method
          1. 8.9.1.4.1 Security and privacy
          2. 8.9.1.4.2 User agent automation
        5. 8.9.1.5 Cookies
        6. 8.9.1.6 PDF viewing support
    10. 8.10 Images
    11. 8.11 Animation frames
  9. 9 Communication
    1. 9.1 The MessageEvent interface
    2. 9.2 Server-sent events
      1. 9.2.1 Introduction
      2. 9.2.2 The EventSource interface
      3. 9.2.3 Processing model
      4. 9.2.4 The `Last-Event-ID` header
      5. 9.2.5 Parsing an event stream
      6. 9.2.6 Interpreting an event stream
      7. 9.2.7 Authoring notes
      8. 9.2.8 Connectionless push and other features
      9. 9.2.9 Garbage collection
      10. 9.2.10 Implementation advice
    3. 9.3 Cross-document messaging
      1. 9.3.1 Introduction
      2. 9.3.2 Security
        1. 9.3.2.1 Authors
        2. 9.3.2.2 User agents
      3. 9.3.3 Posting messages
    4. 9.4 Channel messaging
      1. 9.4.1 Introduction
        1. 9.4.1.1 Examples
        2. 9.4.1.2 Ports as the basis of an object-capability model on the web
        3. 9.4.1.3 Ports as the basis of abstracting out service implementations
      2. 9.4.2 Message channels
      3. 9.4.3 The MessageEventTarget mixin
      4. 9.4.4 Message ports
      5. 9.4.5 Ports and garbage collection
    5. 9.5 Broadcasting to other browsing contexts
  10. 10 Web workers
    1. 10.1 Introduction
      1. 10.1.1 Scope
      2. 10.1.2 Examples
        1. 10.1.2.1 A background number-crunching worker
        2. 10.1.2.2 Using a JavaScript module as a worker
        3. 10.1.2.3 Shared workers introduction
        4. 10.1.2.4 Shared state using a shared worker
        5. 10.1.2.5 Delegation
        6. 10.1.2.6 Providing libraries
      3. 10.1.3 Tutorials
        1. 10.1.3.1 Creating a dedicated worker
        2. 10.1.3.2 Communicating with a dedicated worker
        3. 10.1.3.3 Shared workers
    2. 10.2 Infrastructure
      1. 10.2.1 The global scope
        1. 10.2.1.1 The WorkerGlobalScope common interface
        2. 10.2.1.2 Dedicated workers and the DedicatedWorkerGlobalScope interface
        3. 10.2.1.3 Shared workers and the SharedWorkerGlobalScope interface
      2. 10.2.2 The event loop
      3. 10.2.3 The worker's lifetime
      4. 10.2.4 Processing model
      5. 10.2.5 Runtime script errors
      6. 10.2.6 Creating workers
        1. 10.2.6.1 The AbstractWorker mixin
        2. 10.2.6.2 Script settings for workers
        3. 10.2.6.3 Dedicated workers and the Worker interface
        4. 10.2.6.4 Shared workers and the SharedWorker interface
      7. 10.2.7 Concurrent hardware capabilities
    3. 10.3 APIs available to workers
      1. 10.3.1 Importing scripts and libraries
      2. 10.3.2 The WorkerNavigator interface
      3. 10.3.3 The WorkerLocation interface
  11. 11 Worklets
    1. 11.1 Introduction
      1. 11.1.1 Motivations
      2. 11.1.2 Code idempotence
      3. 11.1.3 Speculative evaluation
    2. 11.2 Examples
      1. 11.2.1 Loading scripts
      2. 11.2.2 Registering a class and invoking its methods
    3. 11.3 Infrastructure
      1. 11.3.1 The global scope
        1. 11.3.1.1 Agents and event loops
        2. 11.3.1.2 Creation and termination
        3. 11.3.1.3 Script settings for worklets
      2. 11.3.2 The Worklet class
      3. 11.3.3 The worklet's lifetime
  12. 12 Web storage
    1. 12.1 Introduction
    2. 12.2 The API
      1. 12.2.1 The Storage interface
      2. 12.2.2 The sessionStorage getter
      3. 12.2.3 The localStorage getter
      4. 12.2.4 The StorageEvent interface
    3. 12.3 Privacy
      1. 12.3.1 User tracking
      2. 12.3.2 Sensitivity of data
    4. 12.4 Security
      1. 12.4.1 DNS spoofing attacks
      2. 12.4.2 Cross-directory attacks
      3. 12.4.3 Implementation risks
  13. 13 The HTML syntax
    1. 13.1 Writing HTML documents
      1. 13.1.1 The DOCTYPE
      2. 13.1.2 Elements
        1. 13.1.2.1 Start tags
        2. 13.1.2.2 End tags
        3. 13.1.2.3 Attributes
        4. 13.1.2.4 Optional tags
        5. 13.1.2.5 Restrictions on content models
        6. 13.1.2.6 Restrictions on the contents of raw text and escapable raw text elements
      3. 13.1.3 Text
        1. 13.1.3.1 Newlines
      4. 13.1.4 Character references
      5. 13.1.5 CDATA sections
      6. 13.1.6 Comments
    2. 13.2 Parsing HTML documents
      1. 13.2.1 Overview of the parsing model
      2. 13.2.2 Parse errors
      3. 13.2.3 The input byte stream
        1. 13.2.3.1 Parsing with a known character encoding
        2. 13.2.3.2 Determining the character encoding
        3. 13.2.3.3 Character encodings
        4. 13.2.3.4 Changing the encoding while parsing
        5. 13.2.3.5 Preprocessing the input stream
      4. 13.2.4 Parse state
        1. 13.2.4.1 The insertion mode
        2. 13.2.4.2 The stack of open elements
        3. 13.2.4.3 The list of active formatting elements
        4. 13.2.4.4 The element pointers
        5. 13.2.4.5 Other parsing state flags
      5. 13.2.5 Tokenization
        1. 13.2.5.1 Data state
        2. 13.2.5.2 RCDATA state
        3. 13.2.5.3 RAWTEXT state
        4. 13.2.5.4 Script data state
        5. 13.2.5.5 PLAINTEXT state
        6. 13.2.5.6 Tag open state
        7. 13.2.5.7 End tag open state
        8. 13.2.5.8 Tag name state
        9. 13.2.5.9 RCDATA less-than sign state
        10. 13.2.5.10 RCDATA end tag open state
        11. 13.2.5.11 RCDATA end tag name state
        12. 13.2.5.12 RAWTEXT less-than sign state
        13. 13.2.5.13 RAWTEXT end tag open state
        14. 13.2.5.14 RAWTEXT end tag name state
        15. 13.2.5.15 Script data less-than sign state
        16. 13.2.5.16 Script data end tag open state
        17. 13.2.5.17 Script data end tag name state
        18. 13.2.5.18 Script data escape start state
        19. 13.2.5.19 Script data escape start dash state
        20. 13.2.5.20 Script data escaped state
        21. 13.2.5.21 Script data escaped dash state
        22. 13.2.5.22 Script data escaped dash dash state
        23. 13.2.5.23 Script data escaped less-than sign state
        24. 13.2.5.24 Script data escaped end tag open state
        25. 13.2.5.25 Script data escaped end tag name state
        26. 13.2.5.26 Script data double escape start state
        27. 13.2.5.27 Script data double escaped state
        28. 13.2.5.28 Script data double escaped dash state
        29. 13.2.5.29 Script data double escaped dash dash state
        30. 13.2.5.30 Script data double escaped less-than sign state
        31. 13.2.5.31 Script data double escape end state
        32. 13.2.5.32 Before attribute name state
        33. 13.2.5.33 Attribute name state
        34. 13.2.5.34 After attribute name state
        35. 13.2.5.35 Before attribute value state
        36. 13.2.5.36 Attribute value (double-quoted) state
        37. 13.2.5.37 Attribute value (single-quoted) state
        38. 13.2.5.38 Attribute value (unquoted) state
        39. 13.2.5.39 After attribute value (quoted) state
        40. 13.2.5.40 Self-closing start tag state
        41. 13.2.5.41 Bogus comment state
        42. 13.2.5.42 Markup declaration open state
        43. 13.2.5.43 Comment start state
        44. 13.2.5.44 Comment start dash state
        45. 13.2.5.45 Comment state
        46. 13.2.5.46 Comment less-than sign state
        47. 13.2.5.47 Comment less-than sign bang state
        48. 13.2.5.48 Comment less-than sign bang dash state
        49. 13.2.5.49 Comment less-than sign bang dash dash state
        50. 13.2.5.50 Comment end dash state
        51. 13.2.5.51 Comment end state
        52. 13.2.5.52 Comment end bang state
        53. 13.2.5.53 DOCTYPE state
        54. 13.2.5.54 Before DOCTYPE name state
        55. 13.2.5.55 DOCTYPE name state
        56. 13.2.5.56 After DOCTYPE name state
        57. 13.2.5.57 After DOCTYPE public keyword state
        58. 13.2.5.58 Before DOCTYPE public identifier state
        59. 13.2.5.59 DOCTYPE public identifier (double-quoted) state
        60. 13.2.5.60 DOCTYPE public identifier (single-quoted) state
        61. 13.2.5.61 After DOCTYPE public identifier state
        62. 13.2.5.62 Between DOCTYPE public and system identifiers state
        63. 13.2.5.63 After DOCTYPE system keyword state
        64. 13.2.5.64 Before DOCTYPE system identifier state
        65. 13.2.5.65 DOCTYPE system identifier (double-quoted) state
        66. 13.2.5.66 DOCTYPE system identifier (single-quoted) state
        67. 13.2.5.67 After DOCTYPE system identifier state
        68. 13.2.5.68 Bogus DOCTYPE state
        69. 13.2.5.69 CDATA section state
        70. 13.2.5.70 CDATA section bracket state
        71. 13.2.5.71 CDATA section end state
        72. 13.2.5.72 Character reference state
        73. 13.2.5.73 Named character reference state
        74. 13.2.5.74 Ambiguous ampersand state
        75. 13.2.5.75 Numeric character reference state
        76. 13.2.5.76 Hexadecimal character reference start state
        77. 13.2.5.77 Decimal character reference start state
        78. 13.2.5.78 Hexadecimal character reference state
        79. 13.2.5.79 Decimal character reference state
        80. 13.2.5.80 Numeric character reference end state
      6. 13.2.6 Tree construction
        1. 13.2.6.1 Creating and inserting nodes
        2. 13.2.6.2 Parsing elements that contain only text
        3. 13.2.6.3 Closing elements that have implied end tags
        4. 13.2.6.4 The rules for parsing tokens in HTML content
          1. 13.2.6.4.1 The "initial" insertion mode
          2. 13.2.6.4.2 The "before html" insertion mode
          3. 13.2.6.4.3 The "before head" insertion mode
          4. 13.2.6.4.4 The "in head" insertion mode
          5. 13.2.6.4.5 The "in head noscript" insertion mode
          6. 13.2.6.4.6 The "after head" insertion mode
          7. 13.2.6.4.7 The "in body" insertion mode
          8. 13.2.6.4.8 The "text" insertion mode
          9. 13.2.6.4.9 The "in table" insertion mode
          10. 13.2.6.4.10 The "in table text" insertion mode
          11. 13.2.6.4.11 The "in caption" insertion mode
          12. 13.2.6.4.12 The "in column group" insertion mode
          13. 13.2.6.4.13 The "in table body" insertion mode
          14. 13.2.6.4.14 The "in row" insertion mode
          15. 13.2.6.4.15 The "in cell" insertion mode
          16. 13.2.6.4.16 The "in select" insertion mode
          17. 13.2.6.4.17 The "in select in table" insertion mode
          18. 13.2.6.4.18 The "in template" insertion mode
          19. 13.2.6.4.19 The "after body" insertion mode
          20. 13.2.6.4.20 The "in frameset" insertion mode
          21. 13.2.6.4.21 The "after frameset" insertion mode
          22. 13.2.6.4.22 The "after after body" insertion mode
          23. 13.2.6.4.23 The "after after frameset" insertion mode
        5. 13.2.6.5 The rules for parsing tokens in foreign content
      7. 13.2.7 The end
      8. 13.2.8 Speculative HTML parsing
      9. 13.2.9 Coercing an HTML DOM into an infoset
      10. 13.2.10 An introduction to error handling and strange cases in the parser
        1. 13.2.10.1 Misnested tags: <b><i></b></i>
        2. 13.2.10.2 Misnested tags: <b><p></b></p>
        3. 13.2.10.3 Unexpected markup in tables
        4. 13.2.10.4 Scripts that modify the page as it is being parsed
        5. 13.2.10.5 The execution of scripts that are moving across multiple documents
        6. 13.2.10.6 Unclosed formatting elements
    3. 13.3 Serializing HTML fragments
    4. 13.4 Parsing HTML fragments
    5. 13.5 Named character references
  14. 14 The XML syntax
    1. 14.1 Writing documents in the XML syntax
    2. 14.2 Parsing XML documents
    3. 14.3 Serializing XML fragments
    4. 14.4 Parsing XML fragments
  15. 15 Rendering
    1. 15.1 Introduction
    2. 15.2 The CSS user agent style sheet and presentational hints
    3. 15.3 Non-replaced elements
      1. 15.3.1 Hidden elements
      2. 15.3.2 The page
      3. 15.3.3 Flow content
      4. 15.3.4 Phrasing content
      5. 15.3.5 Bidirectional text
      6. 15.3.6 Sections and headings
      7. 15.3.7 Lists
      8. 15.3.8 Tables
      9. 15.3.9 Margin collapsing quirks
      10. 15.3.10 Form controls
      11. 15.3.11 The hr element
      12. 15.3.12 The fieldset and legend elements
    4. 15.4 Replaced elements
      1. 15.4.1 Embedded content
      2. 15.4.2 Images
      3. 15.4.3 Attributes for embedded content and images
      4. 15.4.4 Image maps
    5. 15.5 Widgets
      1. 15.5.1 Native appearance
      2. 15.5.2 Writing mode
      3. 15.5.3 Button layout
      4. 15.5.4 The button element
      5. 15.5.5 The details and summary elements
      6. 15.5.6 The input element as a text entry widget
      7. 15.5.7 The input element as domain-specific widgets
      8. 15.5.8 The input element as a range control
      9. 15.5.9 The input element as a color well
      10. 15.5.10 The input element as a checkbox and radio button widgets
      11. 15.5.11 The input element as a file upload control
      12. 15.5.12 The input element as a button
      13. 15.5.13 The marquee element
      14. 15.5.14 The meter element
      15. 15.5.15 The progress element
      16. 15.5.16 The select element
      17. 15.5.17 The textarea element
    6. 15.6 Frames and framesets
    7. 15.7 Interactive media
      1. 15.7.1 Links, forms, and navigation
      2. 15.7.2 The title attribute
      3. 15.7.3 Editing hosts
      4. 15.7.4 Text rendered in native user interfaces
    8. 15.8 Print media
    9. 15.9 Unstyled XML documents
  16. 16 Obsolete features
    1. 16.1 Obsolete but conforming features
      1. 16.1.1 Warnings for obsolete but conforming features
    2. 16.2 Non-conforming features
    3. 16.3 Requirements for implementations
      1. 16.3.1 The marquee element
      2. 16.3.2 Frames
      3. 16.3.3 Other elements, attributes and APIs
  17. 17 IANA considerations
    1. 17.1 text/html
    2. 17.2 multipart/x-mixed-replace
    3. 17.3 application/xhtml+xml
    4. 17.4 text/ping
    5. 17.5 application/microdata+json
    6. 17.6 text/event-stream
    7. 17.7 web+ scheme prefix
  18. Index
    1. Elements
    2. Element content categories
    3. Attributes
    4. Element interfaces
    5. All interfaces
    6. Events
    7. HTTP headers
    8. MIME types
  19. References
  20. Acknowledgments
  21. Intellectual property rights

1 Introduction

1.1 Where does this specification fit?

This specification defines a big part of the web platform, in lots of detail. Its place in the web platform specification stack relative to other specifications can be best summed up as follows:

CSS SVG MathML Service Workers IDB Fetch CSP AV1 Opus PNG THIS SPECIFICATION HTTP TLS DOM Unicode Web IDL MIME URL XML JavaScript Encoding

1.2 Is this HTML5?

This section is non-normative.

In short: Yes.

In more length: the term "HTML5" is widely used as a buzzword to refer to modern web technologies, many of which (though by no means all) are developed at the WHATWG. This document is one such; others are available from the WHATWG Standards overview.

1.3 Background

This section is non-normative.

HTML is the World Wide Web's core markup language. Originally, HTML was primarily designed as a language for semantically describing scientific documents. Its general design, however, has enabled it to be adapted, over the subsequent years, to describe a number of other types of documents and even applications.

1.4 Audience

This section is non-normative.

This specification is intended for authors of documents and scripts that use the features defined in this specification, implementers of tools that operate on pages that use the features defined in this specification, and individuals wishing to establish the correctness of documents or implementations with respect to the requirements of this specification.

This document is probably not suited to readers who do not already have at least a passing familiarity with web technologies, as in places it sacrifices clarity for precision, and brevity for completeness. More approachable tutorials and authoring guides can provide a gentler introduction to the topic.

In particular, familiarity with the basics of DOM is necessary for a complete understanding of some of the more technical parts of this specification. An understanding of Web IDL, HTTP, XML, Unicode, character encodings, JavaScript, and CSS will also be helpful in places but is not essential.

1.5 Scope

This section is non-normative.

This specification is limited to providing a semantic-level markup language and associated semantic-level scripting APIs for authoring accessible pages on the web ranging from static documents to dynamic applications.

The scope of this specification does not include providing mechanisms for media-specific customization of presentation (although default rendering rules for web browsers are included at the end of this specification, and several mechanisms for hooking into CSS are provided as part of the language).

The scope of this specification is not to describe an entire operating system. In particular, hardware configuration software, image manipulation tools, and applications that users would be expected to use with high-end workstations on a daily basis are out of scope. In terms of applications, this specification is targeted specifically at applications that would be expected to be used by users on an occasional basis, or regularly but from disparate locations, with low CPU requirements. Examples of such applications include online purchasing systems, searching systems, games (especially multiplayer online games), public telephone books or address books, communications software (email clients, instant messaging clients, discussion software), document editing software, etc.

1.6 History

This section is non-normative.

For its first five years (1990-1995), HTML went through a number of revisions and experienced a number of extensions, primarily hosted first at CERN, and then at the IETF.

With the creation of the W3C, HTML's development changed venue again. A first abortive attempt at extending HTML in 1995 known as HTML 3.0 then made way to a more pragmatic approach known as HTML 3.2, which was completed in 1997. HTML4 quickly followed later that same year.

The following year, the W3C membership decided to stop evolving HTML and instead begin work on an XML-based equivalent, called XHTML. This effort started with a reformulation of HTML4 in XML, known as XHTML 1.0, which added no new features except the new serialization, and which was completed in 2000. After XHTML 1.0, the W3C's focus turned to making it easier for other working groups to extend XHTML, under the banner of XHTML Modularization. In parallel with this, the W3C also worked on a new language that was not compatible with the earlier HTML and XHTML languages, calling it XHTML2.

Around the time that HTML's evolution was stopped in 1998, parts of the API for HTML developed by browser vendors were specified and published under the name DOM Level 1 (in 1998) and DOM Level 2 Core and DOM Level 2 HTML (starting in 2000 and culminating in 2003). These efforts then petered out, with some DOM Level 3 specifications published in 2004 but the working group being closed before all the Level 3 drafts were completed.

In 2003, the publication of XForms, a technology which was positioned as the next generation of web forms, sparked a renewed interest in evolving HTML itself, rather than finding replacements for it. This interest was borne from the realization that XML's deployment as a web technology was limited to entirely new technologies (like RSS and later Atom), rather than as a replacement for existing deployed technologies (like HTML).

A proof of concept to show that it was possible to extend HTML4's forms to provide many of the features that XForms 1.0 introduced, without requiring browsers to implement rendering engines that were incompatible with existing HTML web pages, was the first result of this renewed interest. At this early stage, while the draft was already publicly available, and input was already being solicited from all sources, the specification was only under Opera Software's copyright.

The idea that HTML's evolution should be reopened was tested at a W3C workshop in 2004, where some of the principles that underlie the HTML5 work (described below), as well as the aforementioned early draft proposal covering just forms-related features, were presented to the W3C jointly by Mozilla and Opera. The proposal was rejected on the grounds that the proposal conflicted with the previously chosen direction for the web's evolution; the W3C staff and membership voted to continue developing XML-based replacements instead.

Shortly thereafter, Apple, Mozilla, and Opera jointly announced their intent to continue working on the effort under the umbrella of a new venue called the WHATWG. A public mailing list was created, and the draft was moved to the WHATWG site. The copyright was subsequently amended to be jointly owned by all three vendors, and to allow reuse of the specification.

The WHATWG was based on several core principles, in particular that technologies need to be backwards compatible, that specifications and implementations need to match even if this means changing the specification rather than the implementations, and that specifications need to be detailed enough that implementations can achieve complete interoperability without reverse-engineering each other.

The latter requirement in particular required that the scope of the HTML5 specification include what had previously been specified in three separate documents: HTML4, XHTML1, and DOM2 HTML. It also meant including significantly more detail than had previously been considered the norm.

In 2006, the W3C indicated an interest to participate in the development of HTML5 after all, and in 2007 formed a working group chartered to work with the WHATWG on the development of the HTML5 specification. Apple, Mozilla, and Opera allowed the W3C to publish the specification under the W3C copyright, while keeping a version with the less restrictive license on the WHATWG site.

For a number of years, both groups then worked together. In 2011, however, the groups came to the conclusion that they had different goals: the W3C wanted to publish a "finished" version of "HTML5", while the WHATWG wanted to continue working on a Living Standard for HTML, continuously maintaining the specification rather than freezing it in a state with known problems, and adding new features as needed to evolve the platform.

In 2019, the WHATWG and W3C signed an agreement to collaborate on a single version of HTML going forward: this document.

1.7 Design notes

This section is non-normative.

It must be admitted that many aspects of HTML appear at first glance to be nonsensical and inconsistent.

HTML, its supporting DOM APIs, as well as many of its supporting technologies, have been developed over a period of several decades by a wide array of people with different priorities who, in many cases, did not know of each other's existence.

Features have thus arisen from many sources, and have not always been designed in especially consistent ways. Furthermore, because of the unique characteristics of the web, implementation bugs have often become de-facto, and now de-jure, standards, as content is often unintentionally written in ways that rely on them before they can be fixed.

Despite all this, efforts have been made to adhere to certain design goals. These are described in the next few subsections.

1.7.1 Serializability of script execution

This section is non-normative.

To avoid exposing web authors to the complexities of multithreading, the HTML and DOM APIs are designed such that no script can ever detect the simultaneous execution of other scripts. Even with workers, the intent is that the behavior of implementations can be thought of as completely serializing the execution of all scripts in all globals.

The exception to this general design principle is the JavaScript SharedArrayBuffer class. Using SharedArrayBuffer objects, it can in fact be observed that scripts in other agents are executing simultaneously. Furthermore, due to the JavaScript memory model, there are situations which not only are un-representable via serialized script execution, but also un-representable via serialized statement execution among those scripts.

1.7.2 Compliance with other specifications

This section is non-normative.

This specification interacts with and relies on a wide variety of other specifications. In certain circumstances, unfortunately, conflicting needs have led to this specification violating the requirements of these other specifications. Whenever this has occurred, the transgressions have each been noted as a "willful violation", and the reason for the violation has been noted.

1.7.3 Extensibility

This section is non-normative.

HTML has a wide array of extensibility mechanisms that can be used for adding semantics in a safe manner:

1.8 HTML vs XML syntax

This section is non-normative.

This specification defines an abstract language for describing documents and applications, and some APIs for interacting with in-memory representations of resources that use this language.

The in-memory representation is known as "DOM HTML", or "the DOM" for short.

There are various concrete syntaxes that can be used to transmit resources that use this abstract language, two of which are defined in this specification.

The first such concrete syntax is the HTML syntax. This is the format suggested for most authors. It is compatible with most legacy web browsers. If a document is transmitted with the text/html MIME type, then it will be processed as an HTML document by web browsers. This specification defines the latest HTML syntax, known simply as "HTML".

The second concrete syntax is XML. When a document is transmitted with an XML MIME type, such as application/xhtml+xml, then it is treated as an XML document by web browsers, to be parsed by an XML processor. Authors are reminded that the processing for XML and HTML differs; in particular, even minor syntax errors will prevent a document labeled as XML from being rendered fully, whereas they would be ignored in the HTML syntax.

The XML syntax for HTML was formerly referred to as "XHTML", but this specification does not use that term (among other reasons, because no such term is used for the HTML syntaxes of MathML and SVG).

The DOM, the HTML syntax, and the XML syntax cannot all represent the same content. For example, namespaces cannot be represented using the HTML syntax, but they are supported in the DOM and in the XML syntax. Similarly, documents that use the noscript feature can be represented using the HTML syntax, but cannot be represented with the DOM or in the XML syntax. Comments that contain the string "-->" can only be represented in the DOM, not in the HTML and XML syntaxes.

1.9 Structure of this specification

This section is non-normative.

This specification is divided into the following major sections:

Introduction
Non-normative materials providing a context for the HTML standard.
Common infrastructure
The conformance classes, algorithms, definitions, and the common underpinnings of the rest of the specification.
Semantics, structure, and APIs of HTML documents
Documents are built from elements. These elements form a tree using the DOM. This section defines the features of this DOM, as well as introducing the features common to all elements, and the concepts used in defining elements.
The elements of HTML
Each element has a predefined meaning, which is explained in this section. Rules for authors on how to use the element, along with user agent requirements for how to handle each element, are also given. This includes large signature features of HTML such as video playback and subtitles, form controls and form submission, and a 2D graphics API known as the HTML canvas.
Microdata
This specification introduces a mechanism for adding machine-readable annotations to documents, so that tools can extract trees of name-value pairs from the document. This section describes this mechanism and some algorithms that can be used to convert HTML documents into other formats. This section also defines some sample Microdata vocabularies for contact information, calendar events, and licensing works.
User interaction
HTML documents can provide a number of mechanisms for users to interact with and modify content, which are described in this section, such as how focus works, and drag-and-drop.
Loading web pages
HTML documents do not exist in a vacuum — this section defines many of the features that affect environments that deal with multiple pages, such as web browsers.
Web application APIs
This section introduces basic features for scripting of applications in HTML.
Web workers
This section defines an API for background threads in JavaScript.
Worklets
This section defines infrastructure for APIs that need to run JavaScript separately from the main JavaScript execution environment.
The communication APIs
This section describes some mechanisms that applications written in HTML can use to communicate with other applications from different domains running on the same client. It also introduces a server-push event stream mechanism known as Server Sent Events or EventSource, and a two-way full-duplex socket protocol for scripts known as Web Sockets.
Web storage
This section defines a client-side storage mechanism based on name-value pairs.
The HTML syntax
The XML syntax
All of these features would be for naught if they couldn't be represented in a serialized form and sent to other people, and so these sections define the syntaxes of HTML and XML, along with rules for how to parse content using those syntaxes.
Rendering
This section defines the default rendering rules for web browsers.

There are also some appendices, listing obsolete features and IANA considerations, and several indices.

1.9.1 How to read this specification

This specification should be read like all other specifications. First, it should be read cover-to-cover, multiple times. Then, it should be read backwards at least once. Then it should be read by picking random sections from the contents list and following all the cross-references.

As described in the conformance requirements section below, this specification describes conformance criteria for a variety of conformance classes. In particular, there are conformance requirements that apply to producers, for example authors and the documents they create, and there are conformance requirements that apply to consumers, for example web browsers. They can be distinguished by what they are requiring: a requirement on a producer states what is allowed, while a requirement on a consumer states how software is to act.

For example, "the foo attribute's value must be a valid integer" is a requirement on producers, as it lays out the allowed values; in contrast, the requirement "the foo attribute's value must be parsed using the rules for parsing integers" is a requirement on consumers, as it describes how to process the content.

Requirements on producers have no bearing whatsoever on consumers.

Continuing the above example, a requirement stating that a particular attribute's value is constrained to being a valid integer emphatically does not imply anything about the requirements on consumers. It might be that the consumers are in fact required to treat the attribute as an opaque string, completely unaffected by whether the value conforms to the requirements or not. It might be (as in the previous example) that the consumers are required to parse the value using specific rules that define how invalid (non-numeric in this case) values are to be processed.

1.9.2 Typographic conventions

This is a definition, requirement, or explanation.

This is a note.

This is an example.

This is an open issue.

This is a warning.

[Exposed=Window]
interface Example {
  // this is an IDL definition
};
variable = object.method([optionalArgument])

This is a note to authors describing the usage of an interface.

/* this is a CSS fragment */

The defining instance of a term is marked up like this. Uses of that term are marked up like this or like this.

The defining instance of an element, attribute, or API is marked up like this. References to that element, attribute, or API are marked up like this.

Other code fragments are marked up like this.

Variables are marked up like this.

In an algorithm, steps in synchronous sections are marked with ⌛.

In some cases, requirements are given in the form of lists with conditions and corresponding requirements. In such cases, the requirements that apply to a condition are always the first set of requirements that follow the condition, even in the case of there being multiple sets of conditions for those requirements. Such cases are presented as follows:

This is a condition
This is another condition
This is the requirement that applies to the conditions above.
This is a third condition
This is the requirement that applies to the third condition.

1.10 A quick introduction to HTML

This section is non-normative.

A basic HTML document looks like this:

<!DOCTYPE html>
<html lang="en">
 <head>
  <title>Sample page</title>
 </head>
 <body>
  <h1>Sample page</h1>
  <p>This is a <a href="demo.html">simple</a> sample.</p>
  <!-- this is a comment -->
 </body>
</html>

HTML documents consist of a tree of elements and text. Each element is denoted in the source by a start tag, such as "<body>", and an end tag, such as "</body>". (Certain start tags and end tags can in certain cases be omitted and are implied by other tags.)

Tags have to be nested such that elements are all completely within each other, without overlapping:

<p>This is <em>very <strong>wrong</em>!</strong></p>
<p>This <em>is <strong>correct</strong>.</em></p>

This specification defines a set of elements that can be used in HTML, along with rules about the ways in which the elements can be nested.

Elements can have attributes, which control how the elements work. In the example below, there is a hyperlink, formed using the a element and its href attribute:

<a href="demo.html">simple</a>

Attributes are placed inside the start tag, and consist of a name and a value, separated by an "=" character. The attribute value can remain unquoted if it doesn't contain ASCII whitespace or any of " ' ` = < or >. Otherwise, it has to be quoted using either single or double quotes. The value, along with the "=" character, can be omitted altogether if the value is the empty string.

<!-- empty attributes -->
<input name=address disabled>
<input name=address disabled="">

<!-- attributes with a value -->
<input name=address maxlength=200>
<input name=address maxlength='200'>
<input name=address maxlength="200">

HTML user agents (e.g., web browsers) then parse this markup, turning it into a DOM (Document Object Model) tree. A DOM tree is an in-memory representation of a document.

DOM trees contain several kinds of nodes, in particular a DocumentType node, Element nodes, Text nodes, Comment nodes, and in some cases ProcessingInstruction nodes.

The markup snippet at the top of this section would be turned into the following DOM tree:

The document element of this tree is the html element, which is the element always found in that position in HTML documents. It contains two elements, head and body, as well as a Text node between them.

There are many more Text nodes in the DOM tree than one would initially expect, because the source contains a number of spaces (represented here by "␣") and line breaks ("⏎") that all end up as Text nodes in the DOM. However, for historical reasons not all of the spaces and line breaks in the original markup appear in the DOM. In particular, all the whitespace before head start tag ends up being dropped silently, and all the whitespace after the body end tag ends up placed at the end of the body.

The head element contains a title element, which itself contains a Text node with the text "Sample page". Similarly, the body element contains an h1 element, a p element, and a comment.


This DOM tree can be manipulated from scripts in the page. Scripts (typically in JavaScript) are small programs that can be embedded using the script element or using event handler content attributes. For example, here is a form with a script that sets the value of the form's output element to say "Hello World":

<form name="main">
 Result: <output name="result"></output>
 <script>
  document.forms.main.elements.result.value = 'Hello World';
 </script>
</form>

Each element in the DOM tree is represented by an object, and these objects have APIs so that they can be manipulated. For instance, a link (e.g. the a element in the tree above) can have its "href" attribute changed in several ways:

var a = document.links[0]; // obtain the first link in the document
a.href = 'sample.html'; // change the destination URL of the link
a.protocol = 'https'; // change just the scheme part of the URL
a.setAttribute('href', 'https://example.com/'); // change the content attribute directly

Since DOM trees are used as the way to represent HTML documents when they are processed and presented by implementations (especially interactive implementations like web browsers), this specification is mostly phrased in terms of DOM trees, instead of the markup described above.


HTML documents represent a media-independent description of interactive content. HTML documents might be rendered to a screen, or through a speech synthesizer, or on a braille display. To influence exactly how such rendering takes place, authors can use a styling language such as CSS.

In the following example, the page has been made yellow-on-blue using CSS.

<!DOCTYPE html>
<html lang="en">
 <head>
  <title>Sample styled page</title>
  <style>
   body { background: navy; color: yellow; }
  </style>
 </head>
 <body>
  <h1>Sample styled page</h1>
  <p>This page is just a demo.</p>
 </body>
</html>

For more details on how to use HTML, authors are encouraged to consult tutorials and guides. Some of the examples included in this specification might also be of use, but the novice author is cautioned that this specification, by necessity, defines the language with a level of detail that might be difficult to understand at first.

1.10.1 Writing secure applications with HTML

This section is non-normative.

When HTML is used to create interactive sites, care needs to be taken to avoid introducing vulnerabilities through which attackers can compromise the integrity of the site itself or of the site's users.

A comprehensive study of this matter is beyond the scope of this document, and authors are strongly encouraged to study the matter in more detail. However, this section attempts to provide a quick introduction to some common pitfalls in HTML application development.

The security model of the web is based on the concept of "origins", and correspondingly many of the potential attacks on the web involve cross-origin actions. [ORIGIN]

Not validating user input
Cross-site scripting (XSS)
SQL injection

When accepting untrusted input, e.g. user-generated content such as text comments, values in URL parameters, messages from third-party sites, etc, it is imperative that the data be validated before use, and properly escaped when displayed. Failing to do this can allow a hostile user to perform a variety of attacks, ranging from the potentially benign, such as providing bogus user information like a negative age, to the serious, such as running scripts every time a user looks at a page that includes the information, potentially propagating the attack in the process, to the catastrophic, such as deleting all data in the server.

When writing filters to validate user input, it is imperative that filters always be safelist-based, allowing known-safe constructs and disallowing all other input. Blocklist-based filters that disallow known-bad inputs and allow everything else are not secure, as not everything that is bad is yet known (for example, because it might be invented in the future).

For example, suppose a page looked at its URL's query string to determine what to display, and the site then redirected the user to that page to display a message, as in:

<ul>
 <li><a href="message.cgi?say=Hello">Say Hello</a>
 <li><a href="message.cgi?say=Welcome">Say Welcome</a>
 <li><a href="message.cgi?say=Kittens">Say Kittens</a>
</ul>

If the message was just displayed to the user without escaping, a hostile attacker could then craft a URL that contained a script element:

https://example.com/message.cgi?say=%3Cscript%3Ealert%28%27Oh%20no%21%27%29%3C/script%3E

If the attacker then convinced a victim user to visit this page, a script of the attacker's choosing would run on the page. Such a script could do any number of hostile actions, limited only by what the site offers: if the site is an e-commerce shop, for instance, such a script could cause the user to unknowingly make arbitrarily many unwanted purchases.

This is called a cross-site scripting attack.

There are many constructs that can be used to try to trick a site into executing code. Here are some that authors are encouraged to consider when writing safelist filters:

Cross-site request forgery (CSRF)

If a site allows a user to make form submissions with user-specific side-effects, for example posting messages on a forum under the user's name, making purchases, or applying for a passport, it is important to verify that the request was made by the user intentionally, rather than by another site tricking the user into making the request unknowingly.

This problem exists because HTML forms can be submitted to other origins.

Sites can prevent such attacks by populating forms with user-specific hidden tokens, or by checking `Origin` headers on all requests.

Clickjacking

A page that provides users with an interface to perform actions that the user might not wish to perform needs to be designed so as to avoid the possibility that users can be tricked into activating the interface.

One way that a user could be so tricked is if a hostile site places the victim site in a small iframe and then convinces the user to click, for instance by having the user play a reaction game. Once the user is playing the game, the hostile site can quickly position the iframe under the mouse cursor just as the user is about to click, thus tricking the user into clicking the victim site's interface.

To avoid this, sites that do not expect to be used in frames are encouraged to only enable their interface if they detect that they are not in a frame (e.g. by comparing the window object to the value of the top attribute).

1.10.2 Common pitfalls to avoid when using the scripting APIs

This section is non-normative.

Scripts in HTML have "run-to-completion" semantics, meaning that the browser will generally run the script uninterrupted before doing anything else, such as firing further events or continuing to parse the document.

On the other hand, parsing of HTML files happens incrementally, meaning that the parser can pause at any point to let scripts run. This is generally a good thing, but it does mean that authors need to be careful to avoid hooking event handlers after the events could have possibly fired.

There are two techniques for doing this reliably: use event handler content attributes, or create the element and add the event handlers in the same script. The latter is safe because, as mentioned earlier, scripts are run to completion before further events can fire.

One way this could manifest itself is with img elements and the load event. The event could fire as soon as the element has been parsed, especially if the image has already been cached (which is common).

Here, the author uses the onload handler on an img element to catch the load event:

<img src="games.png" alt="Games" onload="gamesLogoHasLoaded(event)">

If the element is being added by script, then so long as the event handlers are added in the same script, the event will still not be missed:

<script>
 var img = new Image();
 img.src = 'games.png';
 img.alt = 'Games';
 img.onload = gamesLogoHasLoaded;
 // img.addEventListener('load', gamesLogoHasLoaded, false); // would work also
</script>

However, if the author first created the img element and then in a separate script added the event listeners, there's a chance that the load event would be fired in between, leading it to be missed:

<!-- Do not use this style, it has a race condition! -->
 <img id="games" src="games.png" alt="Games">
 <!-- the 'load' event might fire here while the parser is taking a
      break, in which case you will not see it! -->
 <script>
  var img = document.getElementById('games');
  img.onload = gamesLogoHasLoaded; // might never fire!
 </script>

1.10.3 How to catch mistakes when writing HTML: validators and conformance checkers

This section is non-normative.

Authors are encouraged to make use of conformance checkers (also known as validators) to catch common mistakes. The WHATWG maintains a list of such tools at: https://whatwg.org/validator/

1.11 Conformance requirements for authors

This section is non-normative.

Unlike previous versions of the HTML specification, this specification defines in some detail the required processing for invalid documents as well as valid documents.

However, even though the processing of invalid content is in most cases well-defined, conformance requirements for documents are still important: in practice, interoperability (the situation in which all implementations process particular content in a reliable and identical or equivalent way) is not the only goal of document conformance requirements. This section details some of the more common reasons for still distinguishing between a conforming document and one with errors.

1.11.1 Presentational markup

This section is non-normative.

The majority of presentational features from previous versions of HTML are no longer allowed. Presentational markup in general has been found to have a number of problems:

The use of presentational elements leads to poorer accessibility

While it is possible to use presentational markup in a way that provides users of assistive technologies (ATs) with an acceptable experience (e.g. using ARIA), doing so is significantly more difficult than doing so when using semantically-appropriate markup. Furthermore, even using such techniques doesn't help make pages accessible for non-AT non-graphical users, such as users of text-mode browsers.

Using media-independent markup, on the other hand, provides an easy way for documents to be authored in such a way that they work for more users (e.g. users of text browsers).

Higher cost of maintenance

It is significantly easier to maintain a site written in such a way that the markup is style-independent. For example, changing the color of a site that uses <font color=""> throughout requires changes across the entire site, whereas a similar change to a site based on CSS can be done by changing a single file.

Larger document sizes

Presentational markup tends to be much more redundant, and thus results in larger document sizes.

For those reasons, presentational markup has been removed from HTML in this version. This change should not come as a surprise; HTML4 deprecated presentational markup many years ago and provided a mode (HTML4 Transitional) to help authors move away from presentational markup; later, XHTML 1.1 went further and obsoleted those features altogether.

The only remaining presentational markup features in HTML are the style attribute and the style element. Use of the style attribute is somewhat discouraged in production environments, but it can be useful for rapid prototyping (where its rules can be directly moved into a separate style sheet later) and for providing specific styles in unusual cases where a separate style sheet would be inconvenient. Similarly, the style element can be useful in syndication or for page-specific styles, but in general an external style sheet is likely to be more convenient when the styles apply to multiple pages.

It is also worth noting that some elements that were previously presentational have been redefined in this specification to be media-independent: b, i, hr, s, small, and u.

1.11.2 Syntax errors

This section is non-normative.

The syntax of HTML is constrained to avoid a wide variety of problems.

Unintuitive error-handling behavior

Certain invalid syntax constructs, when parsed, result in DOM trees that are highly unintuitive.

For example, the following markup fragment results in a DOM with an hr element that is an earlier sibling of the corresponding table element:

<table><hr>...
Errors with optional error recovery

To allow user agents to be used in controlled environments without having to implement the more bizarre and convoluted error handling rules, user agents are permitted to fail whenever encountering a parse error.

Errors where the error-handling behavior is not compatible with streaming user agents

Some error-handling behavior, such as the behavior for the <table><hr>... example mentioned above, are incompatible with streaming user agents (user agents that process HTML files in one pass, without storing state). To avoid interoperability problems with such user agents, any syntax resulting in such behavior is considered invalid.

Errors that can result in infoset coercion

When a user agent based on XML is connected to an HTML parser, it is possible that certain invariants that XML enforces, such as element or attribute names never contain multiple colons, will be violated by an HTML file. Handling this can require that the parser coerce the HTML DOM into an XML-compatible infoset. Most syntax constructs that require such handling are considered invalid. (Comments containing two consecutive hyphens, or ending with a hyphen, are exceptions that are allowed in the HTML syntax.)

Errors that result in disproportionately poor performance

Certain syntax constructs can result in disproportionately poor performance. To discourage the use of such constructs, they are typically made non-conforming.

For example, the following markup results in poor performance, since all the unclosed i elements have to be reconstructed in each paragraph, resulting in progressively more elements in each paragraph:

<p><i>She dreamt.
<p><i>She dreamt that she ate breakfast.
<p><i>Then lunch.
<p><i>And finally dinner.

The resulting DOM for this fragment would be:

Errors involving fragile syntax constructs

There are syntax constructs that, for historical reasons, are relatively fragile. To help reduce the number of users who accidentally run into such problems, they are made non-conforming.

For example, the parsing of certain named character references in attributes happens even with the closing semicolon being omitted. It is safe to include an ampersand followed by letters that do not form a named character reference, but if the letters are changed to a string that does form a named character reference, they will be interpreted as that character instead.

In this fragment, the attribute's value is "?bill&ted":

<a href="?bill&ted">Bill and Ted</a>

In the following fragment, however, the attribute's value is actually "?art©", not the intended "?art&copy", because even without the final semicolon, "&copy" is handled the same as "&copy;" and thus gets interpreted as "©":

<a href="?art&copy">Art and Copy</a>

To avoid this problem, all named character references are required to end with a semicolon, and uses of named character references without a semicolon are flagged as errors.

Thus, the correct way to express the above cases is as follows:

<a href="?bill&ted">Bill and Ted</a> <!-- &ted is ok, since it's not a named character reference -->
<a href="?art&amp;copy">Art and Copy</a> <!-- the & has to be escaped, since &copy is a named character reference -->
Errors involving known interoperability problems in legacy user agents

Certain syntax constructs are known to cause especially subtle or serious problems in legacy user agents, and are therefore marked as non-conforming to help authors avoid them.

For example, this is why the U+0060 GRAVE ACCENT character (`) is not allowed in unquoted attributes. In certain legacy user agents, it is sometimes treated as a quote character.

Another example of this is the DOCTYPE, which is required to trigger no-quirks mode, because the behavior of legacy user agents in quirks mode is often largely undocumented.

Errors that risk exposing authors to security attacks

Certain restrictions exist purely to avoid known security problems.

For example, the restriction on using UTF-7 exists purely to avoid authors falling prey to a known cross-site-scripting attack using UTF-7. [UTF7]

Cases where the author's intent is unclear

Markup where the author's intent is very unclear is often made non-conforming. Correcting these errors early makes later maintenance easier.

For example, it is unclear whether the author intended the following to be an h1 heading or an h2 heading:

<h1>Contact details</h2>
Cases that are likely to be typos

When a user makes a simple typo, it is helpful if the error can be caught early, as this can save the author a lot of debugging time. This specification therefore usually considers it an error to use element names, attribute names, and so forth, that do not match the names defined in this specification.

For example, if the author typed <capton> instead of <caption>, this would be flagged as an error and the author could correct the typo immediately.

Errors that could interfere with new syntax in the future

In order to allow the language syntax to be extended in the future, certain otherwise harmless features are disallowed.

For example, "attributes" in end tags are ignored currently, but they are invalid, in case a future change to the language makes use of that syntax feature without conflicting with already-deployed (and valid!) content.

Some authors find it helpful to be in the practice of always quoting all attributes and always including all optional tags, preferring the consistency derived from such custom over the minor benefits of terseness afforded by making use of the flexibility of the HTML syntax. To aid such authors, conformance checkers can provide modes of operation wherein such conventions are enforced.

1.11.3 Restrictions on content models and on attribute values

This section is non-normative.

Beyond the syntax of the language, this specification also places restrictions on how elements and attributes can be specified. These restrictions are present for similar reasons:

Errors involving content with dubious semantics

To avoid misuse of elements with defined meanings, content models are defined that restrict how elements can be nested when such nestings would be of dubious value.

For example, this specification disallows nesting a section element inside a kbd element, since it is highly unlikely for an author to indicate that an entire section should be keyed in.

Errors that involve a conflict in expressed semantics

Similarly, to draw the author's attention to mistakes in the use of elements, clear contradictions in the semantics expressed are also considered conformance errors.

In the fragments below, for example, the semantics are nonsensical: a separator cannot simultaneously be a cell, nor can a radio button be a progress bar.

<hr role="cell">
<input type=radio role=progressbar>

Another example is the restrictions on the content models of the ul element, which only allows li element children. Lists by definition consist just of zero or more list items, so if a ul element contains something other than an li element, it's not clear what was meant.

Cases where the default styles are likely to lead to confusion

Certain elements have default styles or behaviors that make certain combinations likely to lead to confusion. Where these have equivalent alternatives without this problem, the confusing combinations are disallowed.

For example, div elements are rendered as block boxes, and span elements as inline boxes. Putting a block box in an inline box is unnecessarily confusing; since either nesting just div elements, or nesting just span elements, or nesting span elements inside div elements all serve the same purpose as nesting a div element in a span element, but only the latter involves a block box in an inline box, the latter combination is disallowed.

Another example would be the way interactive content cannot be nested. For example, a button element cannot contain a textarea element. This is because the default behavior of such nesting interactive elements would be highly confusing to users. Instead of nesting these elements, they can be placed side by side.

Errors that indicate a likely misunderstanding of the specification

Sometimes, something is disallowed because allowing it would likely cause author confusion.

For example, setting the disabled attribute to the value "false" is disallowed, because despite the appearance of meaning that the element is enabled, it in fact means that the element is disabled (what matters for implementations is the presence of the attribute, not its value).

Errors involving limits that have been imposed merely to simplify the language

Some conformance errors simplify the language that authors need to learn.

For example, the area element's shape attribute, despite accepting both circ and circle values in practice as synonyms, disallows the use of the circ value, so as to simplify tutorials and other learning aids. There would be no benefit to allowing both, but it would cause extra confusion when teaching the language.

Errors that involve peculiarities of the parser

Certain elements are parsed in somewhat eccentric ways (typically for historical reasons), and their content model restrictions are intended to avoid exposing the author to these issues.

For example, a form element isn't allowed inside phrasing content, because when parsed as HTML, a form element's start tag will imply a p element's end tag. Thus, the following markup results in two paragraphs, not one:

<p>Welcome. <form><label>Name:</label> <input></form>

It is parsed exactly like the following:

<p>Welcome. </p><form><label>Name:</label> <input></form>
Errors that would likely result in scripts failing in hard-to-debug ways

Some errors are intended to help prevent script problems that would be hard to debug.

This is why, for instance, it is non-conforming to have two id attributes with the same value. Duplicate IDs lead to the wrong element being selected, with sometimes disastrous effects whose cause is hard to determine.

Errors that waste authoring time

Some constructs are disallowed because historically they have been the cause of a lot of wasted authoring time, and by encouraging authors to avoid making them, authors can save time in future efforts.

For example, a script element's src attribute causes the element's contents to be ignored. However, this isn't obvious, especially if the element's contents appear to be executable script — which can lead to authors spending a lot of time trying to debug the inline script without realizing that it is not executing. To reduce this problem, this specification makes it non-conforming to have executable script in a script element when the src attribute is present. This means that authors who are validating their documents are less likely to waste time with this kind of mistake.

Errors that involve areas that affect authors migrating between the HTML and XML syntaxes

Some authors like to write files that can be interpreted as both XML and HTML with similar results. Though this practice is discouraged in general due to the myriad of subtle complications involved (especially when involving scripting, styling, or any kind of automated serialization), this specification has a few restrictions intended to at least somewhat mitigate the difficulties. This makes it easier for authors to use this as a transitionary step when migrating between the HTML and XML syntaxes.

For example, there are somewhat complicated rules surrounding the lang and xml:lang attributes intended to keep the two synchronized.

Another example would be the restrictions on the values of xmlns attributes in the HTML serialization, which are intended to ensure that elements in conforming documents end up in the same namespaces whether processed as HTML or XML.

Errors that involve areas reserved for future expansion

As with the restrictions on the syntax intended to allow for new syntax in future revisions of the language, some restrictions on the content models of elements and values of attributes are intended to allow for future expansion of the HTML vocabulary.

For example, limiting the values of the target attribute that start with an U+005F LOW LINE character (_) to only specific predefined values allows new predefined values to be introduced at a future time without conflicting with author-defined values.

Errors that indicate a mis-use of other specifications

Certain restrictions are intended to support the restrictions made by other specifications.

For example, requiring that attributes that take media query lists use only valid media query lists reinforces the importance of following the conformance rules of that specification.

1.12 Suggested reading

This section is non-normative.

The following documents might be of interest to readers of this specification.

Character Model for the World Wide Web 1.0: Fundamentals [CHARMOD]

This Architectural Specification provides authors of specifications, software developers, and content developers with a common reference for interoperable text manipulation on the World Wide Web, building on the Universal Character Set, defined jointly by the Unicode Standard and ISO/IEC 10646. Topics addressed include use of the terms 'character', 'encoding' and 'string', a reference processing model, choice and identification of character encodings, character escaping, and string indexing.

Unicode Security Considerations [UTR36]

Because Unicode contains such a large number of characters and incorporates the varied writing systems of the world, incorrect usage can expose programs or systems to possible security attacks. This is especially important as more and more products are internationalized. This document describes some of the security considerations that programmers, system analysts, standards developers, and users should take into account, and provides specific recommendations to reduce the risk of problems.

Web Content Accessibility Guidelines (WCAG) [WCAG]

Web Content Accessibility Guidelines (WCAG) covers a wide range of recommendations for making web content more accessible. Following these guidelines will make content accessible to a wider range of people with disabilities, including blindness and low vision, deafness and hearing loss, learning disabilities, cognitive limitations, limited movement, speech disabilities, photosensitivity and combinations of these. Following these guidelines will also often make your web content more usable to users in general.

Authoring Tool Accessibility Guidelines (ATAG) 2.0 [ATAG]

This specification provides guidelines for designing web content authoring tools that are more accessible for people with disabilities. An authoring tool that conforms to these guidelines will promote accessibility by providing an accessible user interface to authors with disabilities as well as by enabling, supporting, and promoting the production of accessible web content by all authors.

User Agent Accessibility Guidelines (UAAG) 2.0 [UAAG]

This document provides guidelines for designing user agents that lower barriers to web accessibility for people with disabilities. User agents include browsers and other types of software that retrieve and render web content. A user agent that conforms to these guidelines will promote accessibility through its own user interface and through other internal facilities, including its ability to communicate with other technologies (especially assistive technologies). Furthermore, all users, not just users with disabilities, should find conforming user agents to be more usable.

2 Common infrastructure

This specification depends on Infra. [INFRA]

2.1 Terminology

This specification refers to both HTML and XML attributes and IDL attributes, often in the same context. When it is not clear which is being referred to, they are referred to as content attributes for HTML and XML attributes, and IDL attributes for those defined on IDL interfaces. Similarly, the term "properties" is used for both JavaScript object properties and CSS properties. When these are ambiguous they are qualified as object properties and CSS properties respectively.

Generally, when the specification states that a feature applies to the HTML syntax or the XML syntax, it also includes the other. When a feature specifically only applies to one of the two languages, it is called out by explicitly stating that it does not apply to the other format, as in "for HTML, ... (this does not apply to XML)".

This specification uses the term document to refer to any use of HTML, ranging from short static documents to long essays or reports with rich multimedia, as well as to fully-fledged interactive applications. The term is used to refer both to Document objects and their descendant DOM trees, and to serialized byte streams using the HTML syntax or the XML syntax, depending on context.

In the context of the DOM structures, the terms HTML document and XML document are used as defined in DOM, and refer specifically to two different modes that Document objects can find themselves in. [DOM] (Such uses are always hyperlinked to their definition.)

In the context of byte streams, the term HTML document refers to resources labeled as text/html, and the term XML document refers to resources labeled with an XML MIME type.


For simplicity, terms such as shown, displayed, and visible might sometimes be used when referring to the way a document is rendered to the user. These terms are not meant to imply a visual medium; they must be considered to apply to other media in equivalent ways.

2.1.1 Parallelism

To run steps in parallel means those steps are to be run, one after another, at the same time as other logic in the standard (e.g., at the same time as the event loop). This standard does not define the precise mechanism by which this is achieved, be it time-sharing cooperative multitasking, fibers, threads, processes, using different hyperthreads, cores, CPUs, machines, etc. By contrast, an operation that is to run immediately must interrupt the currently running task, run itself, and then resume the previously running task.

For guidance on writing specifications that leverage parallelism, see Dealing with the event loop from other specifications.

To avoid race conditions between different in parallel algorithms that operate on the same data, a parallel queue can be used.

A parallel queue represents a queue of algorithm steps that must be run in series.

A parallel queue has an algorithm queue (a queue), initially empty.

To enqueue steps to a parallel queue, enqueue the algorithm steps to the parallel queue's algorithm queue.

To start a new parallel queue, run the following steps:

  1. Let parallelQueue be a new parallel queue.

  2. Run the following steps in parallel:

    1. While true:

      1. Let steps be the result of dequeueing from parallelQueue's algorithm queue.

      2. If steps is not nothing, then run steps.

      3. Assert: running steps did not throw an exception, as steps running in parallel are not allowed to throw.

      Implementations are not expected to implement this as a continuously running loop. Algorithms in standards are to be easy to understand and are not necessarily great for battery life or performance.

  3. Return parallelQueue.

Steps running in parallel can themselves run other steps in in parallel. E.g., inside a parallel queue it can be useful to run a series of steps in parallel with the queue.

Imagine a standard defined nameList (a list), along with a method to add a name to nameList, unless nameList already contains name, in which case it rejects.

The following solution suffers from race conditions:

  1. Let p be a new promise created in this's relevant realm.

  2. Run the following steps in parallel:

    1. If nameList contains name, then queue a global task on the DOM manipulation task source given this's relevant global object to reject p with a TypeError, and abort these steps.

    2. Do some potentially lengthy work.

    3. Append name to nameList.

    4. Queue a global task on the DOM manipulation task source given this's relevant global object to resolve p with undefined.

  3. Return p.

Two invocations of the above could run simultaneously, meaning name isn't in nameList during step 2.1, but it might be added before step 2.3 runs, meaning name ends up in nameList twice.

Parallel queues solve this. The standard would let nameListQueue be the result of starting a new parallel queue, then:

  1. Let p be a new promise created in this's relevant realm.

  2. Enqueue the following steps to nameListQueue:

    1. If nameList contains name, then queue a global task on the DOM manipulation task source given this's relevant global object to reject p with a TypeError, and abort these steps.

    2. Do some potentially lengthy work.

    3. Append name to nameList.

    4. Queue a global task on the DOM manipulation task source given this's relevant global object to resolve p with undefined.

  3. Return p.

The steps would now queue and the race is avoided.

2.1.2 Resources

The specification uses the term supported when referring to whether a user agent has an implementation capable of decoding the semantics of an external resource. A format or type is said to be supported if the implementation can process an external resource of that format or type without critical aspects of the resource being ignored. Whether a specific resource is supported can depend on what features of the resource's format are in use.

For example, a PNG image would be considered to be in a supported format if its pixel data could be decoded and rendered, even if, unbeknownst to the implementation, the image also contained animation data.

An MPEG-4 video file would not be considered to be in a supported format if the compression format used was not supported, even if the implementation could determine the dimensions of the movie from the file's metadata.

What some specifications, in particular the HTTP specifications, refer to as a representation is referred to in this specification as a resource. [HTTP]

A resource's critical subresources are those that the resource needs to have available to be correctly processed. Which resources are considered critical or not is defined by the specification that defines the resource's format.

For CSS style sheets, we tentatively define here that their critical subresources are other style sheets imported via @import rules, including those indirectly imported by other imported style sheets.

This definition is not fully interoperable; furthermore, some user agents seem to count resources like background images or web fonts as critical subresources. Ideally, the CSS Working Group would define this; see w3c/csswg-drafts issue #1088 to track progress on that front.

2.1.3 XML compatibility

To ease migration from HTML to XML, user agents conforming to this specification will place elements in HTML in the http://www.w3.org/1999/xhtml namespace, at least for the purposes of the DOM and CSS. The term "HTML elements" refers to any element in that namespace, even in XML documents.

Except where otherwise stated, all elements defined or mentioned in this specification are in the HTML namespace ("http://www.w3.org/1999/xhtml"), and all attributes defined or mentioned in this specification have no namespace.

The term element type is used to refer to the set of elements that have a given local name and namespace. For example, button elements are elements with the element type button, meaning they have the local name "button" and (implicitly as defined above) the HTML namespace.

Attribute names are said to be XML-compatible if they match the Name production defined in XML and they contain no U+003A COLON characters (:). [XML]

2.1.4 DOM trees

When it is stated that some element or attribute is ignored, or treated as some other value, or handled as if it was something else, this refers only to the processing of the node after it is in the DOM. A user agent must not mutate the DOM in such situations.

A content attribute is said to change value only if its new value is different than its previous value; setting an attribute to a value it already has does not change it.

The term empty, when used for an attribute value, Text node, or string, means that the length of the text is zero (i.e., not even containing controls or U+0020 SPACE).

An HTML element can have specific HTML element insertion steps, HTML element post-connection steps, and HTML element removing steps, all defined for the element's local name.

The insertion steps for the HTML Standard, given insertedNode, are defined as the following:

  1. If insertedNode is an element whose namespace is the HTML namespace, and this standard defines HTML element insertion steps for insertedNode's local name, then run the corresponding HTML element insertion steps given insertedNode.

  2. If insertedNode is a form-associated element or the ancestor of a form-associated element, then:

    1. If the form-associated element's parser inserted flag is set, then return.

    2. Reset the form owner of the form-associated element.

  3. If insertedNode is an Element that is not on the stack of open elements of an HTML parser, then process internal resource links given insertedNode's node document.

The post-connection steps for the HTML Standard, given insertedNode, are defined as the following:

  1. If insertedNode is an element whose namespace is the HTML namespace, and this standard defines HTML element post-connection steps for insertedNode's local name, then run the corresponding HTML element post-connection steps given insertedNode.

The removing steps for the HTML Standard, given removedNode and oldParent, are defined as the following:

  1. Let document be removedNode's node document.

  2. If document's focused area is removedNode, then set document's focused area to document's viewport, and set document's relevant global object's navigation API's focus changed during ongoing navigation to false.

    This does not perform the unfocusing steps, focusing steps, or focus update steps, and thus no blur or change events are fired.

  3. If removedNode is an element whose namespace is the HTML namespace, and this standard defines HTML element removing steps for removedNode's local name, then run the corresponding HTML element removing steps given removedNode and oldParent.

  4. If removedNode is a form-associated element or the ancestor of a form-associated element, then:

    1. If the form-associated element has a form owner and the form-associated element and its form owner are no longer in the same tree, then reset the form owner of the form-associated element.

  5. If removedNode's popover attribute is not in the no popover state, then run the hide popover algorithm given removedNode, false, false, and false.

A node is inserted into a document when the insertion steps are invoked with it as the argument and it is now in a document tree. Analogously, a node is removed from a document when the removing steps are invoked with it as the argument and it is now no longer in a document tree.

A node becomes connected when the insertion steps are invoked with it as the argument and it is now connected. Analogously, a node becomes disconnected when the removing steps are invoked with it as the argument and it is now no longer connected.

A node is browsing-context connected when it is connected and its shadow-including root's browsing context is non-null. A node becomes browsing-context connected when the insertion steps are invoked with it as the argument and it is now browsing-context connected. A node becomes browsing-context disconnected either when the removing steps are invoked with it as the argument and it is now no longer browsing-context connected, or when its shadow-including root's browsing context becomes null.

2.1.5 Scripting

The construction "a Foo object", where Foo is actually an interface, is sometimes used instead of the more accurate "an object implementing the interface Foo".

An IDL attribute is said to be getting when its value is being retrieved (e.g. by author script), and is said to be setting when a new value is assigned to it.

If a DOM object is said to be live, then the attributes and methods on that object must operate on the actual underlying data, not a snapshot of the data.

2.1.6 Plugins

The term plugin refers to an implementation-defined set of content handlers used by the user agent that can take part in the user agent's rendering of a Document object, but that neither act as child navigables of the Document nor introduce any Node objects to the Document's DOM.

Typically such content handlers are provided by third parties, though a user agent can also designate built-in content handlers as plugins.

A user agent must not consider the types text/plain and application/octet-stream as having a registered plugin.

One example of a plugin would be a PDF viewer that is instantiated in a navigable when the user navigates to a PDF file. This would count as a plugin regardless of whether the party that implemented the PDF viewer component was the same as that which implemented the user agent itself. However, a PDF viewer application that launches separate from the user agent (as opposed to using the same interface) is not a plugin by this definition.

This specification does not define a mechanism for interacting with plugins, as it is expected to be user-agent- and platform-specific. Some UAs might opt to support a plugin mechanism such as the Netscape Plugin API; others might use remote content converters or have built-in support for certain types. Indeed, this specification doesn't require user agents to support plugins at all. [NPAPI]

Browsers should take extreme care when interacting with external content intended for plugins. When third-party software is run with the same privileges as the user agent itself, vulnerabilities in the third-party software become as dangerous as those in the user agent.

(This is a tracking vector.) Since different users having different sets of plugins provides a tracking vector that increases the chances of users being uniquely identified, user agents are encouraged to support the exact same set of plugins for each user.

2.1.7 Character encodings

A character encoding, or just encoding where that is not ambiguous, is a defined way to convert between byte streams and Unicode strings, as defined in Encoding. An encoding has an encoding name and one or more encoding labels, referred to as the encoding's name and labels in the Encoding standard. [ENCODING]

2.1.8 Conformance classes

This specification describes the conformance criteria for user agents (relevant to implementers) and documents (relevant to authors and authoring tool implementers).

Conforming documents are those that comply with all the conformance criteria for documents. For readability, some of these conformance requirements are phrased as conformance requirements on authors; such requirements are implicitly requirements on documents: by definition, all documents are assumed to have had an author. (In some cases, that author may itself be a user agent — such user agents are subject to additional rules, as explained below.)

For example, if a requirement states that "authors must not use the foobar element", it would imply that documents are not allowed to contain elements named foobar.

There is no implied relationship between document conformance requirements and implementation conformance requirements. User agents are not free to handle non-conformant documents as they please; the processing model described in this specification applies to implementations regardless of the conformity of the input documents.

User agents fall into several (overlapping) categories with different conformance requirements.

Web browsers and other interactive user agents

Web browsers that support the XML syntax must process elements and attributes from the HTML namespace found in XML documents as described in this specification, so that users can interact with them, unless the semantics of those elements have been overridden by other specifications.

A conforming web browser would, upon finding a script element in an XML document, execute the script contained in that element. However, if the element is found within a transformation expressed in XSLT (assuming the user agent also supports XSLT), then the processor would instead treat the script element as an opaque element that forms part of the transform.

Web browsers that support the HTML syntax must process documents labeled with an HTML MIME type as described in this specification, so that users can interact with them.

User agents that support scripting must also be conforming implementations of the IDL fragments in this specification, as described in Web IDL. [WEBIDL]

Unless explicitly stated, specifications that override the semantics of HTML elements do not override the requirements on DOM objects representing those elements. For example, the script element in the example above would still implement the HTMLScriptElement interface.

Non-interactive presentation user agents

User agents that process HTML and XML documents purely to render non-interactive versions of them must comply to the same conformance criteria as web browsers, except that they are exempt from requirements regarding user interaction.

Typical examples of non-interactive presentation user agents are printers (static UAs) and overhead displays (dynamic UAs). It is expected that most static non-interactive presentation user agents will also opt to lack scripting support.

A non-interactive but dynamic presentation UA would still execute scripts, allowing forms to be dynamically submitted, and so forth. However, since the concept of "focus" is irrelevant when the user cannot interact with the document, the UA would not need to support any of the focus-related DOM APIs.

Visual user agents that support the suggested default rendering

User agents, whether interactive or not, may be designated (possibly as a user option) as supporting the suggested default rendering defined by this specification.

This is not required. In particular, even user agents that do implement the suggested default rendering are encouraged to offer settings that override this default to improve the experience for the user, e.g. changing the color contrast, using different focus styles, or otherwise making the experience more accessible and usable to the user.

User agents that are designated as supporting the suggested default rendering must, while so designated, implement the rules the Rendering section defines as the behavior that user agents are expected to implement.

User agents with no scripting support

Implementations that do not support scripting (or which have their scripting features disabled entirely) are exempt from supporting the events and DOM interfaces mentioned in this specification. For the parts of this specification that are defined in terms of an events model or in terms of the DOM, such user agents must still act as if events and the DOM were supported.

Scripting can form an integral part of an application. Web browsers that do not support scripting, or that have scripting disabled, might be unable to fully convey the author's intent.

Conformance checkers

Conformance checkers must verify that a document conforms to the applicable conformance criteria described in this specification. Automated conformance checkers are exempt from detecting errors that require interpretation of the author's intent (for example, while a document is non-conforming if the content of a blockquote element is not a quote, conformance checkers running without the input of human judgement do not have to check that blockquote elements only contain quoted material).

Conformance checkers must check that the input document conforms when parsed without a browsing context (meaning that no scripts are run, and that the parser's scripting flag is disabled), and should also check that the input document conforms when parsed with a browsing context in which scripts execute, and that the scripts never cause non-conforming states to occur other than transiently during script execution itself. (This is only a "SHOULD" and not a "MUST" requirement because it has been proven to be impossible. [COMPUTABLE])

The term "HTML validator" can be used to refer to a conformance checker that itself conforms to the applicable requirements of this specification.

XML DTDs cannot express all the conformance requirements of this specification. Therefore, a validating XML processor and a DTD cannot constitute a conformance checker. Also, since neither of the two authoring formats defined in this specification are applications of SGML, a validating SGML system cannot constitute a conformance checker either.

To put it another way, there are three types of conformance criteria:

  1. Criteria that can be expressed in a DTD.
  2. Criteria that cannot be expressed by a DTD, but can still be checked by a machine.
  3. Criteria that can only be checked by a human.

A conformance checker must check for the first two. A simple DTD-based validator only checks for the first class of errors and is therefore not a conforming conformance checker according to this specification.

Data mining tools

Applications and tools that process HTML and XML documents for reasons other than to either render the documents or check them for conformance should act in accordance with the semantics of the documents that they process.

A tool that generates document outlines but increases the nesting level for each paragraph and does not increase the nesting level for headings would not be conforming.

Authoring tools and markup generators

Authoring tools and markup generators must generate conforming documents. Conformance criteria that apply to authors also apply to authoring tools, where appropriate.

Authoring tools are exempt from the strict requirements of using elements only for their specified purpose, but only to the extent that authoring tools are not yet able to determine author intent. However, authoring tools must not automatically misuse elements or encourage their users to do so.

For example, it is not conforming to use an address element for arbitrary contact information; that element can only be used for marking up contact information for its nearest article or body element ancestor. However, since an authoring tool is likely unable to determine the difference, an authoring tool is exempt from that requirement. This does not mean, though, that authoring tools can use address elements for any block of italics text (for instance); it just means that the authoring tool doesn't have to verify that when the user uses a tool for inserting contact information for an article element, that the user really is doing that and not inserting something else instead.

In terms of conformance checking, an editor has to output documents that conform to the same extent that a conformance checker will verify.

When an authoring tool is used to edit a non-conforming document, it may preserve the conformance errors in sections of the document that were not edited during the editing session (i.e. an editing tool is allowed to round-trip erroneous content). However, an authoring tool must not claim that the output is conformant if errors have been so preserved.

Authoring tools are expected to come in two broad varieties: tools that work from structure or semantic data, and tools that work on a What-You-See-Is-What-You-Get media-specific editing basis (WYSIWYG).

The former is the preferred mechanism for tools that author HTML, since the structure in the source information can be used to make informed choices regarding which HTML elements and attributes are most appropriate.

However, WYSIWYG tools are legitimate. WYSIWYG tools should use elements they know are appropriate, and should not use elements that they do not know to be appropriate. This might in certain extreme cases mean limiting the use of flow elements to just a few elements, like div, b, i, and span and making liberal use of the style attribute.

All authoring tools, whether WYSIWYG or not, should make a best effort attempt at enabling users to create well-structured, semantically rich, media-independent content.

For compatibility with existing content and prior specifications, this specification describes two authoring formats: one based on XML, and one using a custom format inspired by SGML (referred to as the HTML syntax). Implementations must support at least one of these two formats, although supporting both is encouraged.

Some conformance requirements are phrased as requirements on elements, attributes, methods or objects. Such requirements fall into two categories: those describing content model restrictions, and those describing implementation behavior. Those in the former category are requirements on documents and authoring tools. Those in the second category are requirements on user agents. Similarly, some conformance requirements are phrased as requirements on authors; such requirements are to be interpreted as conformance requirements on the documents that authors produce. (In other words, this specification does not distinguish between conformance criteria on authors and conformance criteria on documents.)

2.1.9 Dependencies

This specification relies on several other underlying specifications.

Infra

The following terms are defined in Infra: [INFRA]

Unicode and Encoding

The Unicode character set is used to represent textual data, and Encoding defines requirements around character encodings. [UNICODE]

This specification introduces terminology based on the terms defined in those specifications, as described earlier.

The following terms are used as defined in Encoding: [ENCODING]

  • Getting an encoding
  • Get an output encoding
  • The generic decode algorithm which takes a byte stream and an encoding and returns a character stream
  • The UTF-8 decode algorithm which takes a byte stream and returns a character stream, additionally stripping one leading UTF-8 Byte Order Mark (BOM), if any
  • The UTF-8 decode without BOM algorithm which is identical to UTF-8 decode except that it does not strip one leading UTF-8 Byte Order Mark (BOM)
  • The encode algorithm which takes a character stream and an encoding and returns a byte stream
  • The UTF-8 encode algorithm which takes a character stream and returns a byte stream
  • The BOM sniff algorithm which takes a byte stream and returns an encoding or null.
XML and related specifications

Implementations that support the XML syntax for HTML must support some version of XML, as well as its corresponding namespaces specification, because that syntax uses an XML serialization with namespaces. [XML] [XMLNS]

Data mining tools and other user agents that perform operations on content without running scripts, evaluating CSS or XPath expressions, or otherwise exposing the resulting DOM to arbitrary content, may "support namespaces" by just asserting that their DOM node analogues are in certain namespaces, without actually exposing the namespace strings.

In the HTML syntax, namespace prefixes and namespace declarations do not have the same effect as in XML. For instance, the colon has no special meaning in HTML element names.


The attribute with the name space in the XML namespace is defined by Extensible Markup Language (XML). [XML]

The Name production is defined in XML. [XML]

This specification also references the <?xml-stylesheet?> processing instruction, defined in Associating Style Sheets with XML documents. [XMLSSPI]

This specification also non-normatively mentions the XSLTProcessor interface and its transformToFragment() and transformToDocument() methods. [XSLTP]

URLs

The following terms are defined in URL: [URL]

A number of schemes and protocols are referenced by this specification also:

Media fragment syntax is defined in Media Fragments URI. [MEDIAFRAG]

HTTP and related specifications

The following terms are defined in the HTTP specifications: [HTTP]

The following terms are defined in HTTP State Management Mechanism: [COOKIES]

The following term is defined in Web Linking: [WEBLINK]

The following terms are defined in Structured Field Values for HTTP: [STRUCTURED-FIELDS]

The following terms are defined in MIME Sniffing: [MIMESNIFF]

Fetch

The following terms are defined in Fetch: [FETCH]

The following terms are defined in Referrer Policy: [REFERRERPOLICY]

The following terms are defined in Mixed Content: [MIX]

The following terms are defined in Subresource Integrity: [SRI]

Paint Timing

The following terms are defined in Paint Timing: [PAINTTIMING]

Navigation Timing

The following terms are defined in Navigation Timing: [NAVIGATIONTIMING]

Resource Timing

The following terms are defined in Resource Timing: [RESOURCETIMING]

Performance Timeline

The following terms are defined in Performance Timeline: [PERFORMANCETIMELINE]

Long Animation Frames

The following terms are defined in Long Animation Frames: [LONGANIMATIONFRAMES]

Long Tasks

The following terms are defined in Long Tasks: [LONGTASKS]

Web IDL

The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in Web IDL. [WEBIDL]

The following terms are defined in Web IDL:

Web IDL also defines the following types that are used in Web IDL fragments in this specification:

The term throw in this specification is used as defined in Web IDL. The DOMException type and the following exception names are defined by Web IDL and used by this specification:

When this specification requires a user agent to create a Date object representing a particular time (which could be the special value Not-a-Number), the milliseconds component of that time, if any, must be truncated to an integer, and the time value of the newly created Date object must represent the resulting truncated time.

For instance, given the time 23045 millionths of a second after 01:00 UTC on January 1st 2000, i.e. the time 2000-01-01T00:00:00.023045Z, then the Date object created representing that time would represent the same time as that created representing the time 2000-01-01T00:00:00.023Z, 45 millionths earlier. If the given time is NaN, then the result is a Date object that represents a time value NaN (indicating that the object does not represent a specific instant of time).

JavaScript

Some parts of the language described by this specification only support JavaScript as the underlying scripting language. [JAVASCRIPT]

The term "JavaScript" is used to refer to ECMA-262, rather than the official term ECMAScript, since the term JavaScript is more widely known.

The following terms are defined in the JavaScript specification and used in this specification:

Users agents that support JavaScript must also implement the Dynamic Code Brand Checks proposal. The following terms are defined there, and used in this specification: [JSDYNAMICCODEBRANDCHECKS]

Users agents that support JavaScript must also implement ECMAScript Internationalization API. [JSINTL]

User agents that support JavaScript must also implement the Import Attributes proposal. The following terms are defined there, and used in this specification: [JSIMPORTATTRIBUTES]

User agents that support JavaScript must also implement the JSON modules proposal. The following terms are defined there, and used in this specification: [JSJSONMODULES]

User agents that support JavaScript must also implement the Resizable ArrayBuffer and growable SharedArrayBuffer proposal. The following terms are defined there, and used in this specification: [JSRESIZABLEBUFFERS]

User agents that support JavaScript must also implement the Temporal proposal. The following terms are defined there, and used in this specification: [JSTEMPORAL]

WebAssembly

The following term is defined in WebAssembly JavaScript Interface: [WASMJS]

DOM

The Document Object Model (DOM) is a representation — a model — of a document and its content. The DOM is not just an API; the conformance criteria of HTML implementations are defined, in this specification, in terms of operations on the DOM. [DOM]

Implementations must support DOM and the events defined in UI Events, because this specification is defined in terms of the DOM, and some of the features are defined as extensions to the DOM interfaces. [DOM] [UIEVENTS]

In particular, the following features are defined in DOM: [DOM]

The following features are defined in UI Events: [UIEVENTS]

The following features are defined in Touch Events: [TOUCH]

The following features are defined in Pointer Events: [POINTEREVENTS]

The following events are defined in Clipboard API and events: [CLIPBOARD-APIS]

This specification sometimes uses the term name to refer to the event's type; as in, "an event named click" or "if the event name is keypress". The terms "name" and "type" for events are synonymous.

The following features are defined in DOM Parsing and Serialization: [DOMPARSING]

The following features are defined in Selection API: [SELECTION]

User agents are encouraged to implement the features described in execCommand. [EXECCOMMAND]

The following features are defined in Fullscreen API: [FULLSCREEN]

High Resolution Time provides the following features: [HRT]

File API

This specification uses the following features defined in File API: [FILEAPI]

Indexed Database API

The following terms are defined in Indexed Database API: [INDEXEDDB]

Media Source Extensions

The following terms are defined in Media Source Extensions: [MEDIASOURCE]

Media Capture and Streams

The following terms are defined in Media Capture and Streams: [MEDIASTREAM]

Reporting

The following terms are defined in Reporting: [REPORTING]

XMLHttpRequest

The following features and terms are defined in XMLHttpRequest: [XHR]

Battery Status

The following features are defined in Battery Status API: [BATTERY]

Media Queries

Implementations must support Media Queries. The <media-condition> feature is defined therein. [MQ]

CSS modules

While support for CSS as a whole is not required of implementations of this specification (though it is encouraged, at least for web browsers), some features are defined in terms of specific CSS requirements.

When this specification requires that something be parsed according to a particular CSS grammar, the relevant algorithm in CSS Syntax must be followed, including error handling rules. [CSSSYNTAX]

For example, user agents are required to close all open constructs upon finding the end of a style sheet unexpectedly. Thus, when parsing the string "rgb(0,0,0" (with a missing close-parenthesis) for a color value, the close parenthesis is implied by this error handling rule, and a value is obtained (the color 'black'). However, the similar construct "rgb(0,0," (with both a missing parenthesis and a missing "blue" value) cannot be parsed, as closing the open construct does not result in a viable value.

The following terms and features are defined in Cascading Style Sheets (CSS): [CSS]

The basic version of the 'display' property is defined in CSS, and the property is extended by other CSS modules. [CSS] [CSSRUBY] [CSSTABLE]

The following terms and features are defined in CSS Box Model: [CSSBOX]

The following features are defined in CSS Logical Properties: [CSSLOGICAL]

The following terms and features are defined in CSS Color: [CSSCOLOR]

The following terms are defined in CSS Images: [CSSIMAGES]

The term paint source is used as defined in CSS Images Level 4 to define the interaction of certain HTML elements with the CSS 'element()' function. [CSSIMAGES4]

The following features are defined in CSS Backgrounds and Borders: [CSSBG]

CSS Backgrounds and Borders also defines the following border properties: [CSSBG]

Border properties
Top Bottom Left Right
Width 'border-top-width' 'border-bottom-width' 'border-left-width' 'border-right-width'
Style 'border-top-style' 'border-bottom-style' 'border-left-style' 'border-right-style'
Color 'border-top-color' 'border-bottom-color' 'border-left-color' 'border-right-color'

The following features are defined in CSS Box Alignment: [CSSALIGN]

The following terms and features are defined in CSS Display: [CSSDISPLAY]

The following features are defined in CSS Flexible Box Layout: [CSSFLEXBOX]

The following terms and features are defined in CSS Fonts: [CSSFONTS]

The following features are defined in CSS Grid Layout: [CSSGRID]

The following terms are defined in CSS Inline Layout: [CSSINLINE]

The following terms and features are defined in CSS Box Sizing: [CSSSIZING]

The following features are defined in CSS Lists and Counters. [CSSLISTS]

The following features are defined in CSS Overflow. [CSSOVERFLOW]

The following terms and features are defined in CSS Positioned Layout: [CSSPOSITION]

The following features are defined in CSS Multi-column Layout. [CSSMULTICOL]

The 'ruby-base' value of the 'display' property is defined in CSS Ruby Layout. [CSSRUBY]

The following features are defined in CSS Table: [CSSTABLE]

The following features are defined in CSS Text: [CSSTEXT]

The following features are defined in CSS Writing Modes: [CSSWM]

The following features are defined in CSS Basic User Interface: [CSSUI]

The algorithm to update animations and send events is defined in Web Animations. [WEBANIMATIONS]

Implementations that support scripting must support the CSS Object Model. The following features and terms are defined in the CSSOM specifications: [CSSOM] [CSSOMVIEW]

The following features and terms are defined in CSS Syntax: [CSSSYNTAX]

The following terms are defined in Selectors: [SELECTORS]

The following features are defined in CSS Values and Units: [CSSVALUES]

The following features are defined in CSS View Transitions: [CSSVIEWTRANSITIONS]

The term style attribute is defined in CSS Style Attributes. [CSSATTR]

The following terms are defined in the CSS Cascading and Inheritance: [CSSCASCADE]

The CanvasRenderingContext2D object's use of fonts depends on the features described in the CSS Fonts and Font Loading specifications, including in particular FontFace objects and the font source concept. [CSSFONTS] [CSSFONTLOAD]

The following interfaces and terms are defined in Geometry Interfaces: [GEOMETRY]

The following terms are defined in the CSS Scoping: [CSSSCOPING]

The following terms and features are defined in CSS Color Adjustment: [CSSCOLORADJUST]

The following terms are defined in CSS Pseudo-Elements: [CSSPSEUDO]

The following terms are defined in CSS Containment: [CSSCONTAIN]

The following terms are defined in CSS Anchor Positioning: [CSSANCHOR]

Intersection Observer

The following term is defined in Intersection Observer: [INTERSECTIONOBSERVER]

Resize Observer

The following terms are defined in Resize Observer: [RESIZEOBSERVER]

WebGL

The following interfaces are defined in the WebGL specifications: [WEBGL]

WebGPU

The following interfaces are defined in WebGPU: [WEBGPU]

WebVTT

Implementations may support WebVTT as a text track format for subtitles, captions, metadata, etc., for media resources. [WEBVTT]

The following terms, used in this specification, are defined in WebVTT:

ARIA

The role attribute is defined in Accessible Rich Internet Applications (ARIA), as are the following roles: [ARIA]

In addition, the following aria-* content attributes are defined in ARIA: [ARIA]

Finally, the following terms are defined ARIA: [ARIA]

Content Security Policy

The following terms are defined in Content Security Policy: [CSP]

Service Workers

The following terms are defined in Service Workers: [SW]

Secure Contexts

The following algorithms are defined in Secure Contexts: [SECURE-CONTEXTS]

Permissions Policy

The following terms are defined in Permissions Policy: [PERMISSIONSPOLICY]

Payment Request API

The following feature is defined in Payment Request API: [PAYMENTREQUEST]

MathML

While support for MathML as a whole is not required by this specification (though it is encouraged, at least for web browsers), certain features depend upon small parts of MathML being implemented. [MATHML]

The following features are defined in Mathematical Markup Language (MathML):

SVG

While support for SVG as a whole is not required by this specification (though it is encouraged, at least for web browsers), certain features depend upon parts of SVG being implemented.

User agents that implement SVG must implement the SVG 2 specification, and not any earlier revisions.

The following features are defined in the SVG 2 specification: [SVG]

Filter Effects

The following features are defined in Filter Effects: [FILTERS]

Compositing

The following features are defined in Compositing and Blending: [COMPOSITE]

Cooperative Scheduling of Background Tasks

The following features are defined in Cooperative Scheduling of Background Tasks: [REQUESTIDLECALLBACK]

Screen Orientation

The following terms are defined in Screen Orientation: [SCREENORIENTATION]

Storage

The following terms are defined in Storage: [STORAGE]

Web App Manifest

The following features are defined in Web App Manifest: [MANIFEST]

WebAssembly JavaScript Interface: ESM Integration

The following terms are defined in WebAssembly JavaScript Interface: ESM Integration: [WASMESM]

WebCodecs

The following features are defined in WebCodecs: [WEBCODECS]

WebDriver

The following terms are defined in WebDriver: [WEBDRIVER]

WebDriver BiDi

The following terms are defined in WebDriver BiDi: [WEBDRIVERBIDI]

Web Cryptography API

The following terms are defined in Web Cryptography API: [WEBCRYPTO]

WebSockets

The following terms are defined in WebSockets: [WEBSOCKETS]

WebTransport

The following terms are defined in WebTransport: [WEBTRANSPORT]

Web Authentication: An API for accessing Public Key Credentials

The following terms are defined in Web Authentication: An API for accessing Public Key Credentials: [WEBAUTHN]

Credential Management

The following terms are defined in Credential Management: [CREDMAN]

Console

The following terms are defined in Console: [CONSOLE]

Web Locks API

The following terms are defined in Web Locks API: [WEBLOCKS]

Trusted Types

This specification uses the following features defined in Trusted Types: [TRUSTED-TYPES]

WebRTC API

The following terms are defined in WebRTC API: [WEBRTC]

Picture-in-Picture API

The following terms are defined in Picture-in-Picture API: [PICTUREINPICTURE]

Idle Detection API

The following terms are defined in Idle Detection API:

Web Speech API

The following terms are defined in Web Speech API:

WebOTP API

The following terms are defined in WebOTP API:

Web Share API

The following terms are defined in Web Share API:

Web Smart Card API

The following terms are defined in Web Smart Card API:

Web Background Synchronization

The following terms are defined in Web Background Synchronization:

Web Periodic Background Synchronization

The following terms are defined in Web Periodic Background Synchronization:

Web Background Fetch

The following terms are defined in Background Fetch:

Keyboard Lock

The following terms are defined in Keyboard Lock:

Web MIDI API

The following terms are defined in Web MIDI API:

Generic Sensor API

The following terms are defined in Generic Sensor API:

WebHID API

The following terms are defined in WebHID API:

WebXR Device API

The following terms are defined in WebXR Device API:


This specification does not require support of any particular network protocol, style sheet language, scripting language, or any of the DOM specifications beyond those required in the list above. However, the language described by this specification is biased towards CSS as the styling language, JavaScript as the scripting language, and HTTP as the network protocol, and several features assume that those languages and protocols are in use.

A user agent that implements the HTTP protocol must implement HTTP State Management Mechanism (Cookies) as well. [HTTP] [COOKIES]

This specification might have certain additional requirements on character encodings, image formats, audio formats, and video formats in the respective sections.

2.1.10 Extensibility

Vendor-specific proprietary user agent extensions to this specification are strongly discouraged. Documents must not use such extensions, as doing so reduces interoperability and fragments the user base, allowing only users of specific user agents to access the content in question.

All extensions must be defined so that the use of extensions neither contradicts nor causes the non-conformance of functionality defined in the specification.

For example, while strongly discouraged from doing so, an implementation could add a new IDL attribute "typeTime" to a control that returned the time it took the user to select the current value of a control (say). On the other hand, defining a new control that appears in a form's elements array would be in violation of the above requirement, as it would violate the definition of elements given in this specification.


When vendor-neutral extensions to this specification are needed, either this specification can be updated accordingly, or an extension specification can be written that overrides the requirements in this specification. When someone applying this specification to their activities decides that they will recognize the requirements of such an extension specification, it becomes an applicable specification for the purposes of conformance requirements in this specification.

Someone could write a specification that defines any arbitrary byte stream as conforming, and then claim that their random junk is conforming. However, that does not mean that their random junk actually is conforming for everyone's purposes: if someone else decides that that specification does not apply to their work, then they can quite legitimately say that the aforementioned random junk is just that, junk, and not conforming at all. As far as conformance goes, what matters in a particular community is what that community agrees is applicable.


User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them.

When support for a feature is disabled (e.g. as an emergency measure to mitigate a security problem, or to aid in development, or for performance reasons), user agents must act as if they had no support for the feature whatsoever, and as if the feature was not mentioned in this specification. For example, if a particular feature is accessed via an attribute in a Web IDL interface, the attribute itself would be omitted from the objects that implement that interface — leaving the attribute on the object but making it return null or throw an exception is insufficient.

2.1.11 Interactions with XPath and XSLT

Implementations of XPath 1.0 that operate on HTML documents parsed or created in the manners described in this specification (e.g. as part of the document.evaluate() API) must act as if the following edit was applied to the XPath 1.0 specification.

First, remove this paragraph:

A QName in the node test is expanded into an expanded-name using the namespace declarations from the expression context. This is the same way expansion is done for element type names in start and end-tags except that the default namespace declared with xmlns is not used: if the QName does not have a prefix, then the namespace URI is null (this is the same way attribute names are expanded). It is an error if the QName has a prefix for which there is no namespace declaration in the expression context.

Then, insert in its place the following:

A QName in the node test is expanded into an expanded-name using the namespace declarations from the expression context. If the QName has a prefix, then there must be a namespace declaration for this prefix in the expression context, and the corresponding namespace URI is the one that is associated with this prefix. It is an error if the QName has a prefix for which there is no namespace declaration in the expression context.

If the QName has no prefix and the principal node type of the axis is element, then the default element namespace is used. Otherwise, if the QName has no prefix, the namespace URI is null. The default element namespace is a member of the context for the XPath expression. The value of the default element namespace when executing an XPath expression through the DOM3 XPath API is determined in the following way:

  1. If the context node is from an HTML DOM, the default element namespace is "http://www.w3.org/1999/xhtml".
  2. Otherwise, the default element namespace URI is null.

This is equivalent to adding the default element namespace feature of XPath 2.0 to XPath 1.0, and using the HTML namespace as the default element namespace for HTML documents. It is motivated by the desire to have implementations be compatible with legacy HTML content while still supporting the changes that this specification introduces to HTML regarding the namespace used for HTML elements, and by the desire to use XPath 1.0 rather than XPath 2.0.

This change is a willful violation of the XPath 1.0 specification, motivated by desire to have implementations be compatible with legacy content while still supporting the changes that this specification introduces to HTML regarding which namespace is used for HTML elements. [XPATH10]


XSLT 1.0 processors outputting to a DOM when the output method is "html" (either explicitly or via the defaulting rule in XSLT 1.0) are affected as follows:

If the transformation program outputs an element in no namespace, the processor must, prior to constructing the corresponding DOM element node, change the namespace of the element to the HTML namespace, ASCII-lowercase the element's local name, and ASCII-lowercase the names of any non-namespaced attributes on the element.

This requirement is a willful violation of the XSLT 1.0 specification, required because this specification changes the namespaces and case-sensitivity rules of HTML in a manner that would otherwise be incompatible with DOM-based XSLT transformations. (Processors that serialize the output are unaffected.) [XSLT10]


This specification does not specify precisely how XSLT processing interacts with the HTML parser infrastructure (for example, whether an XSLT processor acts as if it puts any elements into a stack of open elements). However, XSLT processors must stop parsing if they successfully complete, and must update the current document readiness first to "interactive" and then to "complete" if they are aborted.


This specification does not specify how XSLT interacts with the navigation algorithm, how it fits in with the event loop, nor how error pages are to be handled (e.g. whether XSLT errors are to replace an incremental XSLT output, or are rendered inline, etc.).

There are also additional non-normative comments regarding the interaction of XSLT and HTML in the script element section, and of XSLT, XPath, and HTML in the template element section.

2.2 Policy-controlled features

Headers/Permissions-Policy/document-domain

Support in one engine only.

Firefox🔰 74+SafariNoChrome🔰 88+
Opera?Edge🔰 88+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome AndroidNoWebView Android?Samsung Internet?Opera Android?

This document defines the following policy-controlled features:

Headers/Feature-Policy/autoplay

Firefox🔰 74+SafariNoChrome64+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

Headers/Permissions-Policy/autoplay

Support in one engine only.

Firefox🔰 74+SafariNoChrome88+
Opera?Edge88+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

2.3 Common microsyntaxes

There are various places in HTML that accept particular data types, such as dates or numbers. This section describes what the conformance criteria for content in those formats is, and how to parse them.

Implementers are strongly urged to carefully examine any third-party libraries they might consider using to implement the parsing of syntaxes described below. For example, date libraries are likely to implement error handling behavior that differs from what is required in this specification, since error-handling behavior is often not defined in specifications that describe date syntaxes similar to those used in this specification, and thus implementations tend to vary greatly in how they handle errors.

2.3.1 Common parser idioms

Some of the micro-parsers described below follow the pattern of having an input variable that holds the string being parsed, and having a position variable pointing at the next character to parse in input.

2.3.2 Boolean attributes

A number of attributes are boolean attributes. The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.

If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name, with no leading or trailing whitespace.

The values "true" and "false" are not allowed on boolean attributes. To represent a false value, the attribute has to be omitted altogether.

Here is an example of a checkbox that is checked and disabled. The checked and disabled attributes are the boolean attributes.

<label><input type=checkbox checked name=cheese disabled> Cheese</label>

This could be equivalently written as this:

<label><input type=checkbox checked=checked name=cheese disabled=disabled> Cheese</label>

You can also mix styles; the following is still equivalent:

<label><input type='checkbox' checked name=cheese disabled=""> Cheese</label>

2.3.3 Keywords and enumerated attributes

Some attributes, called enumerated attributes, take on a finite set of states. The state for such an attribute is derived by combining the attribute's value, a set of keyword/state mappings given in the specification of each attribute, and two possible special states that can also be given in the specification of the attribute. These special states are the invalid value default and the missing value default.

Multiple keywords can map to the same state.

The empty string can be a valid keyword. Note that the missing value default applies only when the attribute is missing, not when it is present with an empty string value.

To determine the state of an attribute, use the following steps:

  1. If the attribute is not specified:

    1. If the attribute has a missing value default state defined, then return that missing value default state.

    2. Otherwise, return no state.

  2. If the attribute's value is an ASCII case-insensitive match for one of the keywords defined for the attribute, then return the state represented by that keyword.

  3. If the attribute has an invalid value default state defined, then return that invalid value default state.

  4. Return no state.

For authoring conformance purposes, if an enumerated attribute is specified, the attribute's value must be an ASCII case-insensitive match for one of the conforming keywords for that attribute, with no leading or trailing whitespace.

For reflection purposes, states which have any keywords mapping to them are said to have a canonical keyword. This is determined as follows:

2.3.4 Numbers

2.3.4.1 Signed integers

A string is a valid integer if it consists of one or more ASCII digits, optionally prefixed with a U+002D HYPHEN-MINUS character (-).

A valid integer without a U+002D HYPHEN-MINUS (-) prefix represents the number that is represented in base ten by that string of digits. A valid integer with a U+002D HYPHEN-MINUS (-) prefix represents the number represented in base ten by the string of digits that follows the U+002D HYPHEN-MINUS, subtracted from zero.

The rules for parsing integers are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either an integer or an error.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Let sign have the value "positive".

  4. Skip ASCII whitespace within input given position.

  5. If position is past the end of input, return an error.

  6. If the character indicated by position (the first character) is a U+002D HYPHEN-MINUS character (-):

    1. Let sign be "negative".
    2. Advance position to the next character.
    3. If position is past the end of input, return an error.

    Otherwise, if the character indicated by position (the first character) is a U+002B PLUS SIGN character (+):

    1. Advance position to the next character. (The "+" is ignored, but it is not conforming.)
    2. If position is past the end of input, return an error.
  7. If the character indicated by position is not an ASCII digit, then return an error.

  8. Collect a sequence of code points that are ASCII digits from input given position, and interpret the resulting sequence as a base-ten integer. Let value be that integer.

  9. If sign is "positive", return value, otherwise return the result of subtracting value from zero.

2.3.4.2 Non-negative integers

A string is a valid non-negative integer if it consists of one or more ASCII digits.

A valid non-negative integer represents the number that is represented in base ten by that string of digits.

The rules for parsing non-negative integers are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either zero, a positive integer, or an error.

  1. Let input be the string being parsed.

  2. Let value be the result of parsing input using the rules for parsing integers.

  3. If value is an error, return an error.

  4. If value is less than zero, return an error.

  5. Return value.

2.3.4.3 Floating-point numbers

A string is a valid floating-point number if it consists of:

  1. Optionally, a U+002D HYPHEN-MINUS character (-).

  2. One or both of the following, in the given order:

    1. A series of one or more ASCII digits.

    2. Both of the following, in the given order:

      1. A single U+002E FULL STOP character (.).

      2. A series of one or more ASCII digits.

  3. Optionally:

    1. Either a U+0065 LATIN SMALL LETTER E character (e) or a U+0045 LATIN CAPITAL LETTER E character (E).

    2. Optionally, a U+002D HYPHEN-MINUS character (-) or U+002B PLUS SIGN character (+).

    3. A series of one or more ASCII digits.

A valid floating-point number represents the number obtained by multiplying the significand by ten raised to the power of the exponent, where the significand is the first number, interpreted as base ten (including the decimal point and the number after the decimal point, if any, and interpreting the significand as a negative number if the whole string starts with a U+002D HYPHEN-MINUS character (-) and the number is not zero), and where the exponent is the number after the E, if any (interpreted as a negative number if there is a U+002D HYPHEN-MINUS character (-) between the E and the number and the number is not zero, or else ignoring a U+002B PLUS SIGN character (+) between the E and the number if there is one). If there is no E, then the exponent is treated as zero.

The Infinity and Not-a-Number (NaN) values are not valid floating-point numbers.

The valid floating-point number concept is typically only used to restrict what is allowed for authors, while the user agent requirements use the rules for parsing floating-point number values below (e.g., the max attribute of the progress element). However, in some cases the user agent requirements include checking if a string is a valid floating-point number (e.g., the value sanitization algorithm for the Number state of the input element, or the parse a srcset attribute algorithm).

The best representation of the number n as a floating-point number is the string obtained from running ToString(n). The abstract operation ToString is not uniquely determined. When there are multiple possible strings that could be obtained from ToString for a particular value, the user agent must always return the same string for that value (though it may differ from the value used by other user agents).

The rules for parsing floating-point number values are as given in the following algorithm. This algorithm must be aborted at the first step that returns something. This algorithm will return either a number or an error.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Let value have the value 1.

  4. Let divisor have the value 1.

  5. Let exponent have the value 1.

  6. Skip ASCII whitespace within input given position.

  7. If position is past the end of input, return an error.

  8. If the character indicated by position is a U+002D HYPHEN-MINUS character (-):

    1. Change value and divisor to −1.
    2. Advance position to the next character.
    3. If position is past the end of input, return an error.

    Otherwise, if the character indicated by position (the first character) is a U+002B PLUS SIGN character (+):

    1. Advance position to the next character. (The "+" is ignored, but it is not conforming.)
    2. If position is past the end of input, return an error.
  9. If the character indicated by position is a U+002E FULL STOP (.), and that is not the last character in input, and the character after the character indicated by position is an ASCII digit, then set value to zero and jump to the step labeled fraction.

  10. If the character indicated by position is not an ASCII digit, then return an error.

  11. Collect a sequence of code points that are ASCII digits from input given position, and interpret the resulting sequence as a base-ten integer. Multiply value by that integer.

  12. If position is past the end of input, jump to the step labeled conversion.
  13. Fraction: If the character indicated by position is a U+002E FULL STOP (.), run these substeps:

    1. Advance position to the next character.

    2. If position is past the end of input, or if the character indicated by position is not an ASCII digit, U+0065 LATIN SMALL LETTER E (e), or U+0045 LATIN CAPITAL LETTER E (E), then jump to the step labeled conversion.

    3. If the character indicated by position is a U+0065 LATIN SMALL LETTER E character (e) or a U+0045 LATIN CAPITAL LETTER E character (E), skip the remainder of these substeps.

    4. Fraction loop: Multiply divisor by ten.

    5. Add the value of the character indicated by position, interpreted as a base-ten digit (0..9) and divided by divisor, to value.
    6. Advance position to the next character.

    7. If position is past the end of input, then jump to the step labeled conversion.

    8. If the character indicated by position is an ASCII digit, jump back to the step labeled fraction loop in these substeps.

  14. If the character indicated by position is U+0065 (e) or a U+0045 (E), then:

    1. Advance position to the next character.

    2. If position is past the end of input, then jump to the step labeled conversion.

    3. If the character indicated by position is a U+002D HYPHEN-MINUS character (-):

      1. Change exponent to −1.
      2. Advance position to the next character.
      3. If position is past the end of input, then jump to the step labeled conversion.

      Otherwise, if the character indicated by position is a U+002B PLUS SIGN character (+):

      1. Advance position to the next character.
      2. If position is past the end of input, then jump to the step labeled conversion.

    4. If the character indicated by position is not an ASCII digit, then jump to the step labeled conversion.

    5. Collect a sequence of code points that are ASCII digits from input given position, and interpret the resulting sequence as a base-ten integer. Multiply exponent by that integer.

    6. Multiply value by ten raised to the exponentth power.

  15. Conversion: Let S be the set of finite IEEE 754 double-precision floating-point values except −0, but with two special values added: 21024 and −21024.

  16. Let rounded-value be the number in S that is closest to value, selecting the number with an even significand if there are two equally close values. (The two special values 21024 and −21024 are considered to have even significands for this purpose.)

  17. If rounded-value is 21024 or −21024, return an error.

  18. Return rounded-value.

2.3.4.4 Percentages and lengths

The rules for parsing dimension values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a number greater than or equal to 0.0, or failure; if a number is returned, then it is further categorized as either a percentage or a length.

  1. Let input be the string being parsed.

  2. Let position be a position variable for input, initially pointing at the start of input.

  3. Skip ASCII whitespace within input given position.

  4. If position is past the end of input or the code point at position within input is not an ASCII digit, then return failure.

  5. Collect a sequence of code points that are ASCII digits from input given position, and interpret the resulting sequence as a base-ten integer. Let value be that number.

  6. If position is past the end of input, then return value as a length.

  7. If the code point at position within input is U+002E (.), then:

    1. Advance position by 1.

    2. If position is past the end of input or the code point at position within input is not an ASCII digit, then return the current dimension value with value, input, and position.

    3. Let divisor have the value 1.

    4. While true:

      1. Multiply divisor by ten.

      2. Add the value of the code point at position within input, interpreted as a base-ten digit (0..9) and divided by divisor, to value.

      3. Advance position by 1.

      4. If position is past the end of input, then return value as a length.

      5. If the code point at position within input is not an ASCII digit, then break.

  8. Return the current dimension value with value, input, and position.

The current dimension value, given value, input, and position, is determined as follows:

  1. If position is past the end of input, then return value as a length.

  2. If the code point at position within input is U+0025 (%), then return value as a percentage.

  3. Return value as a length.

2.3.4.5 Nonzero percentages and lengths

The rules for parsing nonzero dimension values are as given in the following algorithm. When invoked, the steps must be followed in the order given, aborting at the first step that returns a value. This algorithm will return either a number greater than 0.0, or an error; if a number is returned, then it is further categorized as either a percentage or a length.

  1. Let input be the string being parsed.

  2. Let value be the result of parsing input using the rules for parsing dimension values.

  3. If value is an error, return an error.

  4. If value is zero, return an error.

  5. If value is a percentage, return value as a percentage.

  6. Return value as a length.

2.3.4.6 Lists of floating-point numbers

A valid list of floating-point numbers is a number of valid floating-point numbers separated by U+002C COMMA characters, with no other characters (e.g. no ASCII whitespace). In addition, there might be restrictions on the number of floating-point numbers that can be given, or on the range of values allowed.

The rules for parsing a list of floating-point numbers are as follows:

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Let numbers be an initially empty list of floating-point numbers. This list will be the result of this algorithm.

  4. Collect a sequence of code points that are ASCII whitespace, U+002C COMMA, or U+003B SEMICOLON characters from input given position. This skips past any leading delimiters.

  5. While position is not past the end of input:

    1. Collect a sequence of code points that are not ASCII whitespace, U+002C COMMA, U+003B SEMICOLON, ASCII digits, U+002E FULL STOP, or U+002D HYPHEN-MINUS characters from input given position. This skips past leading garbage.

    2. Collect a sequence of code points that are not ASCII whitespace, U+002C COMMA, or U+003B SEMICOLON characters from input given position, and let unparsed number be the result.

    3. Let number be the result of parsing unparsed number using the rules for parsing floating-point number values.

    4. If number is an error, set number to zero.

    5. Append number to numbers.

    6. Collect a sequence of code points that are ASCII whitespace, U+002C COMMA, or U+003B SEMICOLON characters from input given position. This skips past the delimiter.

  6. Return numbers.

2.3.4.7 Lists of dimensions

The rules for parsing a list of dimensions are as follows. These rules return a list of zero or more pairs consisting of a number and a unit, the unit being one of percentage, relative, and absolute.

  1. Let raw input be the string being parsed.

  2. If the last character in raw input is a U+002C COMMA character (,), then remove that character from raw input.

  3. Split the string raw input on commas. Let raw tokens be the resulting list of tokens.

  4. Let result be an empty list of number/unit pairs.

  5. For each token in raw tokens, run the following substeps:

    1. Let input be the token.

    2. Let position be a pointer into input, initially pointing at the start of the string.

    3. Let value be the number 0.

    4. Let unit be absolute.

    5. If position is past the end of input, set unit to relative and jump to the last substep.

    6. If the character at position is an ASCII digit, collect a sequence of code points that are ASCII digits from input given position, interpret the resulting sequence as an integer in base ten, and increment value by that integer.

    7. If the character at position is U+002E (.), then:

      1. Collect a sequence of code points consisting of ASCII whitespace and ASCII digits from input given position. Let s be the resulting sequence.

      2. Remove all ASCII whitespace in s.

      3. If s is not the empty string, then:

        1. Let length be the number of characters in s (after the spaces were removed).

        2. Let fraction be the result of interpreting s as a base-ten integer, and then dividing that number by 10length.

        3. Increment value by fraction.

    8. Skip ASCII whitespace within input given position.

    9. If the character at position is a U+0025 PERCENT SIGN character (%), then set unit to percentage.

      Otherwise, if the character at position is a U+002A ASTERISK character (*), then set unit to relative.

    10. Add an entry to result consisting of the number given by value and the unit given by unit.

  6. Return the list result.

2.3.5 Dates and times

In the algorithms below, the number of days in month month of year year is: 31 if month is 1, 3, 5, 7, 8, 10, or 12; 30 if month is 4, 6, 9, or 11; 29 if month is 2 and year is a number divisible by 400, or if year is a number divisible by 4 but not by 100; and 28 otherwise. This takes into account leap years in the Gregorian calendar. [GREGORIAN]

When ASCII digits are used in the date and time syntaxes defined in this section, they express numbers in base ten.

While the formats described here are intended to be subsets of the corresponding ISO8601 formats, this specification defines parsing rules in much more detail than ISO8601. Implementers are therefore encouraged to carefully examine any date parsing libraries before using them to implement the parsing rules described below; ISO8601 libraries might not parse dates and times in exactly the same manner. [ISO8601]

Where this specification refers to the proleptic Gregorian calendar, it means the modern Gregorian calendar, extrapolated backwards to year 1. A date in the proleptic Gregorian calendar, sometimes explicitly referred to as a proleptic-Gregorian date, is one that is described using that calendar even if that calendar was not in use at the time (or place) in question. [GREGORIAN]

The use of the Gregorian calendar as the wire format in this specification is an arbitrary choice resulting from the cultural biases of those involved in the decision. See also the section discussing date, time, and number formats in forms (for authors), implementation notes regarding localization of form controls, and the time element.

2.3.5.1 Months

A month consists of a specific proleptic-Gregorian date with no time-zone information and no date information beyond a year and a month. [GREGORIAN]

A string is a valid month string representing a year year and month month if it consists of the following components in the given order:

  1. Four or more ASCII digits, representing year, where year > 0
  2. A U+002D HYPHEN-MINUS character (-)
  3. Two ASCII digits, representing the month month, in the range 1 ≤ month ≤ 12

The rules to parse a month string are as follows. This will return either a year and month, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Parse a month component to obtain year and month. If this returns nothing, then fail.

  4. If position is not beyond the end of input, then fail.

  5. Return year and month.

The rules to parse a month component, given an input string and a position, are as follows. This will return either a year and a month, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not at least four characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the year.

  2. If year is not a number greater than zero, then fail.

  3. If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.

  4. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the month.

  5. If month is not a number in the range 1 ≤ month ≤ 12, then fail.

  6. Return year and month.

2.3.5.2 Dates

A date consists of a specific proleptic-Gregorian date with no time-zone information, consisting of a year, a month, and a day. [GREGORIAN]

A string is a valid date string representing a year year, month month, and day day if it consists of the following components in the given order:

  1. A valid month string, representing year and month
  2. A U+002D HYPHEN-MINUS character (-)
  3. Two ASCII digits, representing day, in the range 1 ≤ day ≤ maxday where maxday is the number of days in the month month and year year

The rules to parse a date string are as follows. This will return either a date, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Parse a date component to obtain year, month, and day. If this returns nothing, then fail.

  4. If position is not beyond the end of input, then fail.

  5. Let date be the date with year year, month month, and day day.

  6. Return date.

The rules to parse a date component, given an input string and a position, are as follows. This will return either a year, a month, and a day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Parse a month component to obtain year and month. If this returns nothing, then fail.

  2. Let maxday be the number of days in month month of year year.

  3. If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.

  4. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the day.

  5. If day is not a number in the range 1 ≤ day ≤ maxday, then fail.

  6. Return year, month, and day.

2.3.5.3 Yearless dates

A yearless date consists of a Gregorian month and a day within that month, but with no associated year. [GREGORIAN]

A string is a valid yearless date string representing a month month and a day day if it consists of the following components in the given order:

  1. Optionally, two U+002D HYPHEN-MINUS characters (-)
  2. Two ASCII digits, representing the month month, in the range 1 ≤ month ≤ 12
  3. A U+002D HYPHEN-MINUS character (-)
  4. Two ASCII digits, representing day, in the range 1 ≤ day ≤ maxday where maxday is the number of days in the month month and any arbitrary leap year (e.g. 4 or 2000)

In other words, if the month is "02", meaning February, then the day can be 29, as if the year was a leap year.

The rules to parse a yearless date string are as follows. This will return either a month and a day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Parse a yearless date component to obtain month and day. If this returns nothing, then fail.

  4. If position is not beyond the end of input, then fail.

  5. Return month and day.

The rules to parse a yearless date component, given an input string and a position, are as follows. This will return either a month and a day, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Collect a sequence of code points that are U+002D HYPHEN-MINUS characters (-) from input given position. If the collected sequence is not exactly zero or two characters long, then fail.

  2. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the month.

  3. If month is not a number in the range 1 ≤ month ≤ 12, then fail.

  4. Let maxday be the number of days in month month of any arbitrary leap year (e.g. 4 or 2000).

  5. If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.

  6. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the day.

  7. If day is not a number in the range 1 ≤ day ≤ maxday, then fail.

  8. Return month and day.

2.3.5.4 Times

A time consists of a specific time with no time-zone information, consisting of an hour, a minute, a second, and a fraction of a second.

A string is a valid time string representing an hour hour, a minute minute, and a second second if it consists of the following components in the given order:

  1. Two ASCII digits, representing hour, in the range 0 ≤ hour ≤ 23
  2. A U+003A COLON character (:)
  3. Two ASCII digits, representing minute, in the range 0 ≤ minute ≤ 59
  4. If second is nonzero, or optionally if second is zero:
    1. A U+003A COLON character (:)
    2. Two ASCII digits, representing the integer part of second, in the range 0 ≤ s ≤ 59
    3. If second is not an integer, or optionally if second is an integer:
      1. A U+002E FULL STOP character (.)
      2. One, two, or three ASCII digits, representing the fractional part of second

The second component cannot be 60 or 61; leap seconds cannot be represented.

The rules to parse a time string are as follows. This will return either a time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.

  4. If position is not beyond the end of input, then fail.

  5. Let time be the time with hour hour, minute minute, and second second.

  6. Return time.

The rules to parse a time component, given an input string and a position, are as follows. This will return either an hour, a minute, and a second, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the hour.

  2. If hour is not a number in the range 0 ≤ hour ≤ 23, then fail.
  3. If position is beyond the end of input or if the character at position is not a U+003A COLON character, then fail. Otherwise, move position forwards one character.

  4. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the minute.

  5. If minute is not a number in the range 0 ≤ minute ≤ 59, then fail.
  6. Let second be 0.

  7. If position is not beyond the end of input and the character at position is U+003A (:), then:

    1. Advance position to the next character in input.

    2. If position is beyond the end of input, or at the last character in input, or if the next two characters in input starting at position are not both ASCII digits, then fail.

    3. Collect a sequence of code points that are either ASCII digits or U+002E FULL STOP characters from input given position. If the collected sequence is three characters long, or if it is longer than three characters long and the third character is not a U+002E FULL STOP character, or if it has more than one U+002E FULL STOP character, then fail. Otherwise, interpret the resulting sequence as a base-ten number (possibly with a fractional part). Set second to that number.

    4. If second is not a number in the range 0 ≤ second < 60, then fail.

  8. Return hour, minute, and second.

2.3.5.5 Local dates and times

A local date and time consists of a specific proleptic-Gregorian date, consisting of a year, a month, and a day, and a time, consisting of an hour, a minute, a second, and a fraction of a second, but expressed without a time zone. [GREGORIAN]

A string is a valid local date and time string representing a date and time if it consists of the following components in the given order:

  1. A valid date string representing the date
  2. A U+0054 LATIN CAPITAL LETTER T character (T) or a U+0020 SPACE character
  3. A valid time string representing the time

A string is a valid normalized local date and time string representing a date and time if it consists of the following components in the given order:

  1. A valid date string representing the date
  2. A U+0054 LATIN CAPITAL LETTER T character (T)
  3. A valid time string representing the time, expressed as the shortest possible string for the given time (e.g. omitting the seconds component entirely if the given time is zero seconds past the minute)

The rules to parse a local date and time string are as follows. This will return either a date and time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Parse a date component to obtain year, month, and day. If this returns nothing, then fail.

  4. If position is beyond the end of input or if the character at position is neither a U+0054 LATIN CAPITAL LETTER T character (T) nor a U+0020 SPACE character, then fail. Otherwise, move position forwards one character.

  5. Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.

  6. If position is not beyond the end of input, then fail.

  7. Let date be the date with year year, month month, and day day.

  8. Let time be the time with hour hour, minute minute, and second second.

  9. Return date and time.

2.3.5.6 Time zones

A time-zone offset consists of a signed number of hours and minutes.

A string is a valid time-zone offset string representing a time-zone offset if it consists of either:

This format allows for time-zone offsets from -23:59 to +23:59. Right now, in practice, the range of offsets of actual time zones is -12:00 to +14:00, and the minutes component of offsets of actual time zones is always either 00, 30, or 45. There is no guarantee that this will remain so forever, however, since time zones are used as political footballs and are thus subject to very whimsical policy decisions.

See also the usage notes and examples in the global date and time section below for details on using time-zone offsets with historical times that predate the formation of formal time zones.

The rules to parse a time-zone offset string are as follows. This will return either a time-zone offset, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Parse a time-zone offset component to obtain timezonehours and timezoneminutes. If this returns nothing, then fail.

  4. If position is not beyond the end of input, then fail.

  5. Return the time-zone offset that is timezonehours hours and timezoneminutes minutes from UTC.

The rules to parse a time-zone offset component, given an input string and a position, are as follows. This will return either time-zone hours and time-zone minutes, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. If the character at position is a U+005A LATIN CAPITAL LETTER Z character (Z), then:

    1. Let timezonehours be 0.

    2. Let timezoneminutes be 0.

    3. Advance position to the next character in input.

    Otherwise, if the character at position is either a U+002B PLUS SIGN (+) or a U+002D HYPHEN-MINUS (-), then:

    1. If the character at position is a U+002B PLUS SIGN (+), let sign be "positive". Otherwise, it's a U+002D HYPHEN-MINUS (-); let sign be "negative".

    2. Advance position to the next character in input.

    3. Collect a sequence of code points that are ASCII digits from input given position. Let s be the collected sequence.

    4. If s is exactly two characters long, then:

      1. Interpret s as a base-ten integer. Let that number be the timezonehours.

      2. If position is beyond the end of input or if the character at position is not a U+003A COLON character, then fail. Otherwise, move position forwards one character.

      3. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the timezoneminutes.

      If s is exactly four characters long, then:

      1. Interpret the first two characters of s as a base-ten integer. Let that number be the timezonehours.

      2. Interpret the last two characters of s as a base-ten integer. Let that number be the timezoneminutes.

      Otherwise, fail.

    5. If timezonehours is not a number in the range 0 ≤ timezonehours ≤ 23, then fail.
    6. If sign is "negative", then negate timezonehours.
    7. If timezoneminutes is not a number in the range 0 ≤ timezoneminutes ≤ 59, then fail.
    8. If sign is "negative", then negate timezoneminutes.

    Otherwise, fail.

  2. Return timezonehours and timezoneminutes.

2.3.5.7 Global dates and times

A global date and time consists of a specific proleptic-Gregorian date, consisting of a year, a month, and a day, and a time, consisting of an hour, a minute, a second, and a fraction of a second, expressed with a time-zone offset, consisting of a signed number of hours and minutes. [GREGORIAN]

A string is a valid global date and time string representing a date, time, and a time-zone offset if it consists of the following components in the given order:

  1. A valid date string representing the date
  2. A U+0054 LATIN CAPITAL LETTER T character (T) or a U+0020 SPACE character
  3. A valid time string representing the time
  4. A valid time-zone offset string representing the time-zone offset

Times in dates before the formation of UTC in the mid-twentieth century must be expressed and interpreted in terms of UT1 (contemporary Earth solar time at the 0° longitude), not UTC (the approximation of UT1 that ticks in SI seconds). Time before the formation of time zones must be expressed and interpreted as UT1 times with explicit time zones that approximate the contemporary difference between the appropriate local time and the time observed at the location of Greenwich, London.

The following are some examples of dates written as valid global date and time strings.

"0037-12-13 00:00Z"
Midnight in areas using London time on the birthday of Nero (the Roman Emperor). See below for further discussion on which date this actually corresponds to.
"1979-10-14T12:00:00.001-04:00"
One millisecond after noon on October 14th 1979, in the time zone in use on the east coast of the USA during daylight saving time.
"8592-01-01T02:09+02:09"
Midnight UTC on the 1st of January, 8592. The time zone associated with that time is two hours and nine minutes ahead of UTC, which is not currently a real time zone, but is nonetheless allowed.

Several things are notable about these dates:

The rules to parse a global date and time string are as follows. This will return either a time in UTC, with associated time-zone offset information for round-tripping or display purposes, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Parse a date component to obtain year, month, and day. If this returns nothing, then fail.

  4. If position is beyond the end of input or if the character at position is neither a U+0054 LATIN CAPITAL LETTER T character (T) nor a U+0020 SPACE character, then fail. Otherwise, move position forwards one character.

  5. Parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.

  6. If position is beyond the end of input, then fail.

  7. Parse a time-zone offset component to obtain timezonehours and timezoneminutes. If this returns nothing, then fail.

  8. If position is not beyond the end of input, then fail.

  9. Let time be the moment in time at year year, month month, day day, hours hour, minute minute, second second, subtracting timezonehours hours and timezoneminutes minutes. That moment in time is a moment in the UTC time zone.

  10. Let timezone be timezonehours hours and timezoneminutes minutes from UTC.

  11. Return time and timezone.

2.3.5.8 Weeks

A week consists of a week-year number and a week number representing a seven-day period starting on a Monday. Each week-year in this calendaring system has either 52 or 53 such seven-day periods, as defined below. The seven-day period starting on the Gregorian date Monday December 29th 1969 (1969-12-29) is defined as week number 1 in week-year 1970. Consecutive weeks are numbered sequentially. The week before the number 1 week in a week-year is the last week in the previous week-year, and vice versa. [GREGORIAN]

A week-year with a number year has 53 weeks if it corresponds to either a year year in the proleptic Gregorian calendar that has a Thursday as its first day (January 1st), or a year year in the proleptic Gregorian calendar that has a Wednesday as its first day (January 1st) and where year is a number divisible by 400, or a number divisible by 4 but not by 100. All other week-years have 52 weeks.

The week number of the last day of a week-year with 53 weeks is 53; the week number of the last day of a week-year with 52 weeks is 52.

The week-year number of a particular day can be different than the number of the year that contains that day in the proleptic Gregorian calendar. The first week in a week-year y is the week that contains the first Thursday of the Gregorian year y.

For modern purposes, a week as defined here is equivalent to ISO weeks as defined in ISO 8601. [ISO8601]

A string is a valid week string representing a week-year year and week week if it consists of the following components in the given order:

  1. Four or more ASCII digits, representing year, where year > 0
  2. A U+002D HYPHEN-MINUS character (-)
  3. A U+0057 LATIN CAPITAL LETTER W character (W)
  4. Two ASCII digits, representing the week week, in the range 1 ≤ week ≤ maxweek, where maxweek is the week number of the last day of week-year year

The rules to parse a week string are as follows. This will return either a week-year number and week number, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not at least four characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the year.

  4. If year is not a number greater than zero, then fail.

  5. If position is beyond the end of input or if the character at position is not a U+002D HYPHEN-MINUS character, then fail. Otherwise, move position forwards one character.

  6. If position is beyond the end of input or if the character at position is not a U+0057 LATIN CAPITAL LETTER W character (W), then fail. Otherwise, move position forwards one character.

  7. Collect a sequence of code points that are ASCII digits from input given position. If the collected sequence is not exactly two characters long, then fail. Otherwise, interpret the resulting sequence as a base-ten integer. Let that number be the week.

  8. Let maxweek be the week number of the last day of year year.

  9. If week is not a number in the range 1 ≤ week ≤ maxweek, then fail.

  10. If position is not beyond the end of input, then fail.

  11. Return the week-year number year and the week number week.

2.3.5.9 Durations

A duration consists of a number of seconds.

Since months and seconds are not comparable (a month is not a precise number of seconds, but is instead a period whose exact length depends on the precise day from which it is measured) a duration as defined in this specification cannot include months (or years, which are equivalent to twelve months). Only durations that describe a specific number of seconds can be described.

A string is a valid duration string representing a duration t if it consists of either of the following:

The rules to parse a duration string are as follows. This will return either a duration or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Let months, seconds, and component count all be zero.

  4. Let M-disambiguator be minutes.

    This flag's other value is months. It is used to disambiguate the "M" unit in ISO8601 durations, which use the same unit for months and minutes. Months are not allowed, but are parsed for future compatibility and to avoid misinterpreting ISO8601 durations that would be valid in other contexts.

  5. Skip ASCII whitespace within input given position.

  6. If position is past the end of input, then fail.

  7. If the character in input pointed to by position is a U+0050 LATIN CAPITAL LETTER P character, then advance position to the next character, set M-disambiguator to months, and skip ASCII whitespace within input given position.

  8. While true:

    1. Let units be undefined. It will be assigned one of the following values: years, months, weeks, days, hours, minutes, and seconds.

    2. Let next character be undefined. It is used to process characters from the input.

    3. If position is past the end of input, then break.

    4. If the character in input pointed to by position is a U+0054 LATIN CAPITAL LETTER T character, then advance position to the next character, set M-disambiguator to minutes, skip ASCII whitespace within input given position, and continue.

    5. Set next character to the character in input pointed to by position.

    6. If next character is a U+002E FULL STOP character (.), then let N equal zero. (Do not advance position. That is taken care of below.)

      Otherwise, if next character is an ASCII digit, then collect a sequence of code points that are ASCII digits from input given position, interpret the resulting sequence as a base-ten integer, and let N be that number.

      Otherwise, next character is not part of a number; fail.

    7. If position is past the end of input, then fail.

    8. Set next character to the character in input pointed to by position, and this time advance position to the next character. (If next character was a U+002E FULL STOP character (.) before, it will still be that character this time.)

    9. If next character is U+002E (.), then:

      1. Collect a sequence of code points that are ASCII digits from input given position. Let s be the resulting sequence.

      2. If s is the empty string, then fail.

      3. Let length be the number of characters in s.

      4. Let fraction be the result of interpreting s as a base-ten integer, and then dividing that number by 10length.

      5. Increment N by fraction.

      6. Skip ASCII whitespace within input given position.

      7. If position is past the end of input, then fail.

      8. Set next character to the character in input pointed to by position, and advance position to the next character.

      9. If next character is neither a U+0053 LATIN CAPITAL LETTER S character nor a U+0073 LATIN SMALL LETTER S character, then fail.

      10. Set units to seconds.

      Otherwise:

      1. If next character is ASCII whitespace, then skip ASCII whitespace within input given position, set next character to the character in input pointed to by position, and advance position to the next character.

      2. If next character is a U+0059 LATIN CAPITAL LETTER Y character, or a U+0079 LATIN SMALL LETTER Y character, set units to years and set M-disambiguator to months.

        If next character is a U+004D LATIN CAPITAL LETTER M character or a U+006D LATIN SMALL LETTER M character, and M-disambiguator is months, then set units to months.

        If next character is a U+0057 LATIN CAPITAL LETTER W character or a U+0077 LATIN SMALL LETTER W character, set units to weeks and set M-disambiguator to minutes.

        If next character is a U+0044 LATIN CAPITAL LETTER D character or a U+0064 LATIN SMALL LETTER D character, set units to days and set M-disambiguator to minutes.

        If next character is a U+0048 LATIN CAPITAL LETTER H character or a U+0068 LATIN SMALL LETTER H character, set units to hours and set M-disambiguator to minutes.

        If next character is a U+004D LATIN CAPITAL LETTER M character or a U+006D LATIN SMALL LETTER M character, and M-disambiguator is minutes, then set units to minutes.

        If next character is a U+0053 LATIN CAPITAL LETTER S character or a U+0073 LATIN SMALL LETTER S character, set units to seconds and set M-disambiguator to minutes.

        Otherwise, if next character is none of the above characters, then fail.

    10. Increment component count.

    11. Let multiplier be 1.

    12. If units is years, multiply multiplier by 12 and set units to months.

    13. If units is months, add the product of N and multiplier to months.

      Otherwise:

      1. If units is weeks, multiply multiplier by 7 and set units to days.

      2. If units is days, multiply multiplier by 24 and set units to hours.

      3. If units is hours, multiply multiplier by 60 and set units to minutes.

      4. If units is minutes, multiply multiplier by 60 and set units to seconds.

      5. Forcibly, units is now seconds. Add the product of N and multiplier to seconds.

    14. Skip ASCII whitespace within input given position.

  9. If component count is zero, fail.

  10. If months is not zero, fail.

  11. Return the duration consisting of seconds seconds.

2.3.5.10 Vaguer moments in time

A string is a valid date string with optional time if it is also one of the following:


The rules to parse a date or time string are as follows. The algorithm will return either a date, a time, a global date and time, or nothing. If at any point the algorithm says that it "fails", this means that it is aborted at that point and returns nothing.

  1. Let input be the string being parsed.

  2. Let position be a pointer into input, initially pointing at the start of the string.

  3. Set start position to the same position as position.

  4. Set the date present and time present flags to true.

  5. Parse a date component to obtain year, month, and day. If this fails, then set the date present flag to false.

  6. If date present is true, and position is not beyond the end of input, and the character at position is either a U+0054 LATIN CAPITAL LETTER T character (T) or a U+0020 SPACE character, then advance position to the next character in input.

    Otherwise, if date present is true, and either position is beyond the end of input or the character at position is neither a U+0054 LATIN CAPITAL LETTER T character (T) nor a U+0020 SPACE character, then set time present to false.

    Otherwise, if date present is false, set position back to the same position as start position.

  7. If the time present flag is true, then parse a time component to obtain hour, minute, and second. If this returns nothing, then fail.

  8. If the date present and time present flags are both true, but position is beyond the end of input, then fail.

  9. If the date present and time present flags are both true, parse a time-zone offset component to obtain timezonehours and timezoneminutes. If this returns nothing, then fail.

  10. If position is not beyond the end of input, then fail.

  11. If the date present flag is true and the time present flag is false, then let date be the date with year year, month month, and day day, and return date.

    Otherwise, if the time present flag is true and the date present flag is false, then let time be the time with hour hour, minute minute, and second second, and return time.

    Otherwise, let time be the moment in time at year year, month month, day day, hours hour, minute minute, second second, subtracting timezonehours hours and timezoneminutes minutes, that moment in time being a moment in the UTC time zone; let timezone be timezonehours hours and timezoneminutes minutes from UTC; and return time and timezone.

2.3.6 Legacy colors

Some obsolete legacy attributes parse colors using the rules for parsing a legacy color value, given a string input. They will return either a CSS color or failure.

  1. If input is the empty string, then return failure.

  2. Strip leading and trailing ASCII whitespace from input.

  3. If input is an ASCII case-insensitive match for "transparent", then return failure.

  4. If input is an ASCII case-insensitive match for one of the named colors, then return the CSS color corresponding to that keyword. [CSSCOLOR]

    CSS2 System Colors are not recognized.

  5. If input's code point length is four, and the first character in input is U+0023 (#), and the last three characters of input are all ASCII hex digits, then:

    1. Let result be a CSS color.

    2. Interpret the second character of input as a hexadecimal digit; let the red component of result be the resulting number multiplied by 17.

    3. Interpret the third character of input as a hexadecimal digit; let the green component of result be the resulting number multiplied by 17.

    4. Interpret the fourth character of input as a hexadecimal digit; let the blue component of result be the resulting number multiplied by 17.

    5. Return result.

  6. Replace any code points greater than U+FFFF in input (i.e., any characters that are not in the basic multilingual plane) with "00".

  7. If input's code point length is greater than 128, truncate input, leaving only the first 128 characters.

  8. If the first character in input is U+0023 (#), then remove it.

  9. Replace any character in input that is not an ASCII hex digit with U+0030 (0).

  10. While input's code point length is zero or not a multiple of three, append U+0030 (0) to input.

  11. Split input into three strings of equal code point length, to obtain three components. Let length be the code point length that all of those components have (one third the code point length of input).

  12. If length is greater than 8, then remove the leading length-8 characters in each component, and let length be 8.

  13. While length is greater than two and the first character in each component is U+0030 (0), remove that character and reduce length by one.

  14. If length is still greater than two, truncate each component, leaving only the first two characters in each.

  15. Let result be a CSS color.

  16. Interpret the first component as a hexadecimal number; let the red component of result be the resulting number.

  17. Interpret the second component as a hexadecimal number; let the green component of result be the resulting number.

  18. Interpret the third component as a hexadecimal number; let the blue component of result be the resulting number.

  19. Return result.

2.3.7 Space-separated tokens

A set of space-separated tokens is a string containing zero or more words (known as tokens) separated by one or more ASCII whitespace, where words consist of any string of one or more characters, none of which are ASCII whitespace.

A string containing a set of space-separated tokens may have leading or trailing ASCII whitespace.

An unordered set of unique space-separated tokens is a set of space-separated tokens where none of the tokens are duplicated.

An ordered set of unique space-separated tokens is a set of space-separated tokens where none of the tokens are duplicated but where the order of the tokens is meaningful.

Sets of space-separated tokens sometimes have a defined set of allowed values. When a set of allowed values is defined, the tokens must all be from that list of allowed values; other values are non-conforming. If no such set of allowed values is provided, then all values are conforming.

How tokens in a set of space-separated tokens are to be compared (e.g. case-sensitively or not) is defined on a per-set basis.

2.3.8 Comma-separated tokens

A set of comma-separated tokens is a string containing zero or more tokens each separated from the next by a single U+002C COMMA character (,), where tokens consist of any string of zero or more characters, neither beginning nor ending with ASCII whitespace, nor containing any U+002C COMMA characters (,), and optionally surrounded by ASCII whitespace.

For instance, the string " a ,b,,d d " consists of four tokens: "a", "b", the empty string, and "d d". Leading and trailing whitespace around each token doesn't count as part of the token, and the empty string can be a token.

Sets of comma-separated tokens sometimes have further restrictions on what consists a valid token. When such restrictions are defined, the tokens must all fit within those restrictions; other values are non-conforming. If no such restrictions are specified, then all values are conforming.

2.3.9 References

A valid hash-name reference to an element of type type is a string consisting of a U+0023 NUMBER SIGN character (#) followed by a string which exactly matches the value of the name attribute of an element with type type in the same tree.

The rules for parsing a hash-name reference to an element of type type, given a context node scope, are as follows:

  1. If the string being parsed does not contain a U+0023 NUMBER SIGN character, or if the first such character in the string is the last character in the string, then return null.

  2. Let s be the string from the character immediately after the first U+0023 NUMBER SIGN character in the string being parsed up to the end of that string.

  3. Return the first element of type type in scope's tree, in tree order, that has an id or name attribute whose value is s, or null if there is no such element.

    Although id attributes are accounted for when parsing, they are not used in determining whether a value is a valid hash-name reference. That is, a hash-name reference that refers to an element based on id is a conformance error (unless that element also has a name attribute with the same value).

2.3.10 Media queries

A string is a valid media query list if it matches the <media-query-list> production of Media Queries. [MQ]

A string matches the environment of the user if it is the empty string, a string consisting of only ASCII whitespace, or is a media query list that matches the user's environment according to the definitions given in Media Queries. [MQ]

2.3.11 Unique internal values

A unique internal value is a value that is serializable, comparable by value, and never exposed to script.

To create a new unique internal value, return a unique internal value that has never previously been returned by this algorithm.

2.4 URLs

2.4.1 Terminology

A string is a valid non-empty URL if it is a valid URL string but it is not the empty string.

A string is a valid URL potentially surrounded by spaces if, after stripping leading and trailing ASCII whitespace from it, it is a valid URL string.

A string is a valid non-empty URL potentially surrounded by spaces if, after stripping leading and trailing ASCII whitespace from it, it is a valid non-empty URL.

This specification defines the URL about:legacy-compat as a reserved, though unresolvable, about: URL, for use in DOCTYPEs in HTML documents when needed for compatibility with XML tools. [ABOUT]

This specification defines the URL about:html-kind as a reserved, though unresolvable, about: URL, that is used as an identifier for kinds of media tracks. [ABOUT]

This specification defines the URL about:srcdoc as a reserved, though unresolvable, about: URL, that is used as the URL of iframe srcdoc documents. [ABOUT]

The fallback base URL of a Document object document is the URL record obtained by running these steps:

  1. If document is an iframe srcdoc document, then:

    1. Assert: document's about base URL is non-null.

    2. Return document's about base URL.

  2. If document's URL matches about:blank and document's about base URL is non-null, then return document's about base URL.

  3. Return document's URL.

The document base URL of a Document object is the URL record obtained by running these steps:

  1. If there is no base element that has an href attribute in the Document, then return the Document's fallback base URL.

  2. Otherwise, return the frozen base URL of the first base element in the Document that has an href attribute, in tree order.


A URL matches about:blank if its scheme is "about", its path contains a single string "blank", its username and password are the empty string, and its host is null.

Such a URL's query and fragment can be non-null. For example, the URL record created by parsing "about:blank?foo#bar" matches about:blank.

A URL matches about:srcdoc if its scheme is "about", its path contains a single string "srcdoc", its query is null, its username and password are the empty string, and its host is null.

The reason that matches about:srcdoc ensures that the URL's query is null is because it is not possible to create an iframe srcdoc document whose URL has a non-null query, unlike Documents whose URL matches about:blank. In other words, the set of all URLs that match about:srcdoc only vary in their fragment.

2.4.2 Parsing URLs

Parsing a URL is the process of taking a string and obtaining the URL record that it represents. While this process is defined in URL, the HTML standard defines several wrappers to abstract base URLs and encodings. [URL]

Most new APIs are to use parse a URL. Older APIs and HTML elements might have reason to use encoding-parse a URL. When a custom base URL is needed or no base URL is desired, the URL parser can of course be used directly as well.

To parse a URL, given a string url, relative to a Document object or environment settings object environment, run these steps. They return failure or a URL.

  1. Let baseURL be environment's base URL, if environment is a Document object; otherwise environment's API base URL.

  2. Return the result of applying the URL parser to url, with baseURL.

To encoding-parse a URL, given a string url, relative to a Document object or environment settings object environment, run these steps. They return failure or a URL.

  1. Let encoding be UTF-8.

  2. If environment is a Document object, then set encoding to environment's character encoding.

  3. Otherwise, if environment's relevant global object is a Window object, set encoding to environment's relevant global object's associated Document's character encoding.

  4. Let baseURL be environment's base URL, if environment is a Document object; otherwise environment's API base URL.

  5. Return the result of applying the URL parser to url, with baseURL and encoding.

To encoding-parse-and-serialize a URL, given a string url, relative to a Document object or environment settings object environment, run these steps. They return failure or a string.

  1. Let url be the result of encoding-parsing a URL given url, relative to environment.

  2. If url is failure, then return failure.

  3. Return the result of applying the URL serializer to url.

2.4.3 Dynamic changes to base URLs

When a document's document base URL changes, all elements in that document are affected by a base URL change.

The following are base URL change steps, which run when an element is affected by a base URL change (as defined by DOM):

If the element creates a hyperlink

If the URL identified by the hyperlink is being shown to the user, or if any data derived from that URL is affecting the display, then the href attribute's value should be reparsed, relative to the element's node document and the UI updated appropriately.

For example, the CSS :link/:visited pseudo-classes might have been affected.

If the hyperlink has a ping attribute and its URL(s) are being shown to the user, then the ping attribute's tokens should be reparsed, relative to the element's node document and the UI updated appropriately.

If the element is a q, blockquote, ins, or del element with a cite attribute

If the URL identified by the cite attribute is being shown to the user, or if any data derived from that URL is affecting the display, then the cite attribute's value should be reparsed, relative to the element's node document and the UI updated appropriately.

Otherwise

The element is not directly affected.

For instance, changing the base URL doesn't affect the image displayed by img elements, although subsequent accesses of the src IDL attribute from script will return a new absolute URL that might no longer correspond to the image being shown.

2.5 Fetching resources

2.5.1 Terminology

A response whose type is "basic", "cors", or "default" is CORS-same-origin. [FETCH]

A response whose type is "opaque" or "opaqueredirect" is CORS-cross-origin.

A response's unsafe response is its internal response if it has one, and the response itself otherwise.

To create a potential-CORS request, given a url, destination, corsAttributeState, and an optional same-origin fallback flag, run these steps:

  1. Let mode be "no-cors" if corsAttributeState is No CORS, and "cors" otherwise.

  2. If same-origin fallback flag is set and mode is "no-cors", set mode to "same-origin".

  3. Let credentialsMode be "include".

  4. If corsAttributeState is Anonymous, set credentialsMode to "same-origin".

  5. Let request be a new request whose URL is url, destination is destination, mode is mode, credentials mode is credentialsMode, and whose use-URL-credentials flag is set.

2.5.2 Determining the type of a resource

The Content-Type metadata of a resource must be obtained and interpreted in a manner consistent with the requirements of MIME Sniffing. [MIMESNIFF]

The computed MIME type of a resource must be found in a manner consistent with the requirements given in MIME Sniffing. [MIMESNIFF]

The rules for sniffing images specifically, the rules for distinguishing if a resource is text or binary, and the rules for sniffing audio and video specifically are also defined in MIME Sniffing. These rules return a MIME type as their result. [MIMESNIFF]

It is imperative that the rules in MIME Sniffing be followed exactly. When a user agent uses different heuristics for content type detection than the server expects, security problems can occur. For more details, see MIME Sniffing. [MIMESNIFF]

2.5.3 Extracting character encodings from meta elements

The algorithm for extracting a character encoding from a meta element, given a string s, is as follows. It either returns a character encoding or nothing.

  1. Let position be a pointer into s, initially pointing at the start of the string.

  2. Loop: Find the first seven characters in s after position that are an ASCII case-insensitive match for the word "charset". If no such match is found, return nothing.

  3. Skip any ASCII whitespace that immediately follow the word "charset" (there might not be any).

  4. If the next character is not a U+003D EQUALS SIGN (=), then move position to point just before that next character, and jump back to the step labeled loop.

  5. Skip any ASCII whitespace that immediately follow the equals sign (there might not be any).

  6. Process the next character as follows:

    If it is a U+0022 QUOTATION MARK character (") and there is a later U+0022 QUOTATION MARK character (") in s
    If it is a U+0027 APOSTROPHE character (') and there is a later U+0027 APOSTROPHE character (') in s
    Return the result of getting an encoding from the substring that is between this character and the next earliest occurrence of this character.
    If it is an unmatched U+0022 QUOTATION MARK character (")
    If it is an unmatched U+0027 APOSTROPHE character (')
    If there is no next character
    Return nothing.
    Otherwise
    Return the result of getting an encoding from the substring that consists of this character up to but not including the first ASCII whitespace or U+003B SEMICOLON character (;), or the end of s, whichever comes first.

This algorithm is distinct from those in the HTTP specifications (for example, HTTP doesn't allow the use of single quotes and requires supporting a backslash-escape mechanism that is not supported by this algorithm). While the algorithm is used in contexts that, historically, were related to HTTP, the syntax as supported by implementations diverged some time ago. [HTTP]

2.5.4 CORS settings attributes

Attributes/crossorigin

Support in all current engines.

Firefox8+Safari6+Chrome13+
Opera?Edge79+
Edge (Legacy)12+Internet ExplorerYes
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

A CORS settings attribute is an enumerated attribute with the following keywords and states:

Keyword State Brief description
anonymous Anonymous Requests for the element will have their mode set to "cors" and their credentials mode set to "same-origin".
(the empty string)
use-credentials Use Credentials Requests for the element will have their mode set to "cors" and their credentials mode set to "include".

The attribute's missing value default is the No CORS state, and its invalid value default is the Anonymous state. For the purposes of reflection, the canonical keyword for the Anonymous state is the anonymous keyword.

The majority of fetches governed by CORS settings attributes will be done via the create a potential-CORS request algorithm.

For more modern features, where the request's mode is always "cors", certain CORS settings attributes have been repurposed to have a slightly different meaning, wherein they only impact the request's credentials mode. To perform this translation, we define the CORS settings attribute credentials mode for a given CORS settings attribute to be determined by switching on the attribute's state:

No CORS
Anonymous
"same-origin"
Use Credentials
"include"

2.5.5 Referrer policy attributes

A referrer policy attribute is an enumerated attribute. Each referrer policy, including the empty string, is a keyword for this attribute, mapping to a state of the same name.

The attribute's missing value default and invalid value default are both the empty string state.

The impact of these states on the processing model of various fetches is defined in more detail throughout this specification, in Fetch, and in Referrer Policy. [FETCH] [REFERRERPOLICY]

Several signals can contribute to which processing model is used for a given fetch; a referrer policy attribute is only one of them. In general, the order in which these signals are processed are:

  1. First, the presence of a noreferrer link type;

  2. Then, the value of a referrer policy attribute;

  3. Then, the presence of any meta element with name attribute set to referrer.

  4. Finally, the `Referrer-Policy` HTTP header.

2.5.6 Nonce attributes

Global_attributes/nonce

Support in all current engines.

Firefox31+SafariYesChromeYes
Opera?EdgeYes
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

A nonce content attribute represents a cryptographic nonce ("number used once") which can be used by Content Security Policy to determine whether or not a given fetch will be allowed to proceed. The value is text. [CSP]

Elements that have a nonce content attribute ensure that the cryptographic nonce is only exposed to script (and not to side-channels like CSS attribute selectors) by taking the value from the content attribute, moving it into an internal slot named [[CryptographicNonce]], exposing it to script via the HTMLOrSVGElement interface mixin, and setting the content attribute to the empty string. Unless otherwise specified, the slot's value is the empty string.

element.nonce

Returns the value set for element's cryptographic nonce. If the setter was not used, this will be the value originally found in the nonce content attribute.

element.nonce = value

Updates element's cryptographic nonce value.

HTMLElement/nonce

Firefox75+Safari🔰 10+Chrome61+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

The nonce IDL attribute must, on getting, return the value of this element's [[CryptographicNonce]]; and on setting, set this element's [[CryptographicNonce]] to the given value.

Note how the setter for the nonce IDL attribute does not update the corresponding content attribute. This, as well as the below setting of the nonce content attribute to the empty string when an element becomes browsing-context connected, is meant to prevent exfiltration of the nonce value through mechanisms that can easily read content attributes, such as selectors. Learn more in issue #2369, where this behavior was introduced.

The following attribute change steps are used for the nonce content attribute:

  1. If element does not include HTMLOrSVGElement, then return.

  2. If localName is not nonce or namespace is not null, then return.

  3. If value is null, then set element's [[CryptographicNonce]] to the empty string.

  4. Otherwise, set element's [[CryptographicNonce]] to value.

Whenever an element including HTMLOrSVGElement becomes browsing-context connected, the user agent must execute the following steps on the element:

  1. Let CSP list be element's shadow-including root's policy container's CSP list.

  2. If CSP list contains a header-delivered Content Security Policy, and element has a nonce content attribute attr whose value is not the empty string, then:

    1. Let nonce be element's [[CryptographicNonce]].

    2. Set an attribute value for element using "nonce" and the empty string.

    3. Set element's [[CryptographicNonce]] to nonce.

    If element's [[CryptographicNonce]] were not restored it would be the empty string at this point.

The cloning steps for elements that include HTMLOrSVGElement must set the [[CryptographicNonce]] slot on the copy to the value of the slot on the element being cloned.

2.5.7 Lazy loading attributes

Lazy_loading

Support in all current engines.

Firefox75+Safari15.4+Chrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

A lazy loading attribute is an enumerated attribute with the following keywords and states:

Keyword State Brief description
lazy Lazy Used to defer fetching a resource until some conditions are met.
eager Eager Used to fetch a resource immediately; the default state.

The attribute directs the user agent to fetch a resource immediately or to defer fetching until some conditions associated with the element are met, according to the attribute's current state.

The attribute's missing value default and invalid value default are both the Eager state.


The will lazy load element steps, given an element element, are as follows:

  1. If scripting is disabled for element, then return false.

    This is an anti-tracking measure, because if a user agent supported lazy loading when scripting is disabled, it would still be possible for a site to track a user's approximate scroll position throughout a session, by strategically placing images in a page's markup such that a server can track how many images are requested and when.

  2. If element's lazy loading attribute is in the Lazy state, then return true.

  3. Return false.

Each img and iframe element has associated lazy load resumption steps, initially null.

For img and iframe elements that will lazy load, these steps are run from the lazy load intersection observer's callback or when their lazy loading attribute is set to the Eager state. This causes the element to continue loading.

Each Document has a lazy load intersection observer, initially set to null but can be set to an IntersectionObserver instance.

To start intersection-observing a lazy loading element element, run these steps:

  1. Let doc be element's node document.

  2. If doc's lazy load intersection observer is null, set it to a new IntersectionObserver instance, initialized as follows:

    The intention is to use the original value of the IntersectionObserver constructor. However, we're forced to use the JavaScript-exposed constructor in this specification, until Intersection Observer exposes low-level hooks for use in specifications. See bug w3c/IntersectionObserver#464 which tracks this. [INTERSECTIONOBSERVER]

  3. Call doc's lazy load intersection observer's observe method with element as the argument.

    The intention is to use the original value of the observe method. See w3c/IntersectionObserver#464. [INTERSECTIONOBSERVER]

To stop intersection-observing a lazy loading element element, run these steps:

  1. Let doc be element's node document.

  2. Assert: doc's lazy load intersection observer is not null.

  3. Call doc's lazy load intersection observer's unobserve method with element as the argument.

    The intention is to use the original value of the unobserve method. See w3c/IntersectionObserver#464. [INTERSECTIONOBSERVER]

(This is a tracking vector.) The lazy load scroll margin is an implementation-defined value, but with the following suggestions to consider:

It is important for privacy that the lazy load scroll margin not leak additional information. For example, the typical scrolling speed on the current device could be imprecise so as to not introduce a new fingerprinting vector.

2.5.8 Blocking attributes

A blocking attribute explicitly indicates that certain operations should be blocked on the fetching of an external resource. The operations that can be blocked are represented by possible blocking tokens, which are strings listed by the following table:

Possible blocking token Description
"render" The element is potentially render-blocking.

In the future, there might be more possible blocking tokens.

A blocking attribute must have a value that is an unordered set of unique space-separated tokens, each of which are possible blocking tokens. The supported tokens of a blocking attribute are the possible blocking tokens. Any element can have at most one blocking attribute.

The blocking tokens set for an element el are the result of the following steps:

  1. Let value be the value of el's blocking attribute, or the empty string if no such attribute exists.

  2. Set value to value, converted to ASCII lowercase.

  3. Let rawTokens be the result of splitting value on ASCII whitespace.

  4. Return a set containing the elements of rawTokens that are possible blocking tokens.

An element is potentially render-blocking if its blocking tokens set contains "render", or if it is implicitly potentially render-blocking, which will be defined at the individual elements. By default, an element is not implicitly potentially render-blocking.

2.5.9 Fetch priority attributes

A fetch priority attribute is an enumerated attribute with the following keywords and states:

Keyword State Brief description
high high Signals a high-priority fetch relative to other resources with the same destination.
low low Signals a low-priority fetch relative to other resources with the same destination.
auto auto Signals automatic determination of fetch priority relative to other resources with the same destination.

The attribute's missing value default and invalid value default are both the auto state.

2.6 Common DOM interfaces

2.6.1 Reflecting content attributes in IDL attributes

The building blocks for reflecting are as follows:

A reflected IDL attribute can be defined to reflect a reflected content attribute name of a reflected target. In general this means that the IDL attribute getter returns the current value of the content attribute, and the setter changes the value of the content attribute to the given value.

If the reflected target is an element, then the reflected IDL attribute can additionally declare to support ElementInternals. This means that the ElementInternals interface also has a reflected IDL attribute, with the same identifier, and that reflected IDL attribute reflects the same reflected content attribute name.

The fooBar IDL attribute must reflect the foobar content attribute and support ElementInternals.

Reflected targets have these associated algorithms:

For a reflected target that is an element element, these are defined as follows:

get the element
  1. Return element.

get the content attribute
  1. Let attribute be the result of running get an attribute by namespace and local name given null, the reflected content attribute name, and element.

  2. If attribute is null, then return null.

  3. Return attribute's value.

set the content attribute with a string value
  1. Set an attribute value given element, the reflected content attribute name, and value.

delete the content attribute
  1. Remove an attribute by namespace and local name given null, the reflected content attribute name, and element.

For a reflected target that is an ElementInternals object elementInternals, they are defined as follows:

get the element
  1. Return elementInternals's target element.

get the content attribute
  1. If elementInternals's target element's internal content attribute map[the reflected content attribute name] does not exist, then return null.

  2. Return elementInternals's target element's internal content attribute map[the reflected content attribute name].

set the content attribute with a string value
  1. Set elementInternals's target element's internal content attribute map[the reflected content attribute name] to value.

delete the content attribute
  1. Remove elementInternals's target element's internal content attribute map[the reflected content attribute name].

This results in somewhat redundant data structures for ElementInternals objects as their target element's internal content attribute map cannot be directly manipulated and as such reflection is only happening in a single direction. This approach was nevertheless chosen to make it less error-prone to define IDL attributes that are shared between reflected targets and benefit from common API semantics.


IDL attributes of type DOMString or DOMString? that reflect enumerated content attributes can be limited to only known values. Per the processing models below, those will cause the getters for such IDL attributes to only return keywords for those enumerated attributes, or the empty string or null.

If a reflected IDL attribute has the type DOMString:

If a reflected IDL attribute has the type DOMString?:

If a reflected IDL attribute has the type USVString:

If a reflected IDL attribute has the type boolean:

This corresponds to the rules for boolean content attributes.

If a reflected IDL attribute has the type long, optionally limited to only non-negative numbers and optionally with a default value defaultValue:

If a reflected IDL attribute has the type unsigned long, optionally limited to only positive numbers, limited to only positive numbers with fallback, or clamped to the range [clampedMin, clampedMax], and optionally with a default value defaultValue:

If a reflected IDL attribute has the type double, optionally limited to only positive numbers and optionally with a default value defaultValue:

The values Infinity and Not-a-Number (NaN) values throw an exception on setting, as defined in Web IDL. [WEBIDL]

If a reflected IDL attribute has the type DOMTokenList, then its getter steps are to return a DOMTokenList object whose associated element is this and associated attribute's local name is the reflected content attribute name. Specification authors cannot use support ElementInternals for IDL attributes of this type.

If a reflected IDL attribute has the type T?, where T is either Element or an interface that inherits from Element, then with attr being the reflected content attribute name:

Reflected IDL attributes of this type are strongly encouraged to have their identifier end in "Element" for consistency.

If a reflected IDL attribute has the type FrozenArray<T>?, where T is either Element or an interface that inherits from Element, then with attr being the reflected content attribute name:

Reflected IDL attributes of this type are strongly encouraged to have their identifier end in "Elements" for consistency.

2.6.2 Using reflect in specifications

Reflection is primarily about improving web developer ergonomics by giving them typed access to content attributes through reflected IDL attributes. The ultimate source of truth, which the web platform builds upon, is the content attributes themselves. That is, specification authors must not use the reflected IDL attribute getter or setter steps, but instead must use the content attribute presence and value. (Or an abstraction on top, such as the state of an enumerated attribute.)

Two important exceptions to this are reflected IDL attributes whose type is one of the following:

For those, specification authors must use the reflected target's get the attr-associated element and get the attr-associated elements, respectively. The content attribute presence and value must not be used as they cannot be fully synchronized with the reflected IDL attribute.

A reflected target's explicitly set attr-element, explicitly set attr-elements, cached attr-associated elements, and cached attr-associated elements object are to be treated as internal implementation details and not to be built upon.

2.6.3 Collections

The HTMLFormControlsCollection and HTMLOptionsCollection interfaces are collections derived from the HTMLCollection interface. The HTMLAllCollection interface is a collection, but is not so derived.

2.6.3.1 The HTMLAllCollection interface

The HTMLAllCollection interface is used for the legacy document.all attribute. It operates similarly to HTMLCollection; the main differences are that it allows a staggering variety of different (ab)uses of its methods to all end up returning something, and that it can be called as a function as an alternative to property access.

All HTMLAllCollection objects are rooted at a Document and have a filter that matches all elements, so the elements represented by the collection of an HTMLAllCollection object consist of all the descendant elements of the root Document.

Objects that implement the HTMLAllCollection interface are legacy platform objects with an additional [[Call]] internal method described in the section below. They also have an [[IsHTMLDDA]] internal slot.

Objects that implement the HTMLAllCollection interface have several unusual behaviors, due of the fact that they have an [[IsHTMLDDA]] internal slot:

These special behaviors are motivated by a desire for compatibility with two classes of legacy content: one that uses the presence of document.all as a way to detect legacy user agents, and one that only supports those legacy user agents and uses the document.all object without testing for its presence first. [JAVASCRIPT]

[Exposed=Window,
 LegacyUnenumerableNamedProperties]
interface HTMLAllCollection {
  readonly attribute unsigned long length;
  getter Element (unsigned long index);
  getter (HTMLCollection or Element)? namedItem(DOMString name);
  (HTMLCollection or Element)? item(optional DOMString nameOrIndex);

  // Note: HTMLAllCollection objects have a custom [[Call]] internal method and an [[IsHTMLDDA]] internal slot.
};

The object's supported property indices are as defined for HTMLCollection objects.

The supported property names consist of the non-empty values of all the id attributes of all the elements represented by the collection, and the non-empty values of all the name attributes of all the "all"-named elements represented by the collection, in tree order, ignoring later duplicates, with the id of an element preceding its name if it contributes both, they differ from each other, and neither is the duplicate of an earlier entry.

The length getter steps are to return the number of nodes represented by the collection.

The indexed property getter must return the result of getting the "all"-indexed element from this given the passed index.

The namedItem(name) method steps are to return the result of getting the "all"-named element(s) from this given name.

The item(nameOrIndex) method steps are:

  1. If nameOrIndex was not provided, return null.

  2. Return the result of getting the "all"-indexed or named element(s) from this, given nameOrIndex.


The following elements are "all"-named elements: a, button, embed, form, frame, frameset, iframe, img, input, map, meta, object, select, and textarea

To get the "all"-indexed element from an HTMLAllCollection collection given an index index, return the indexth element in collection, or null if there is no such indexth element.

To get the "all"-named element(s) from an HTMLAllCollection collection given a name name, perform the following steps:

  1. If name is the empty string, return null.

  2. Let subCollection be an HTMLCollection object rooted at the same Document as collection, whose filter matches only elements that are either:

  3. If there is exactly one element in subCollection, then return that element.

  4. Otherwise, if subCollection is empty, return null.

  5. Otherwise, return subCollection.

To get the "all"-indexed or named element(s) from an HTMLAllCollection collection given nameOrIndex:

  1. If nameOrIndex, converted to a JavaScript String value, is an array index property name, return the result of getting the "all"-indexed element from collection given the number represented by nameOrIndex.

  2. Return the result of getting the "all"-named element(s) from collection given nameOrIndex.

2.6.3.1.1 [[Call]] ( thisArgument, argumentsList )
  1. If argumentsList's size is zero, or if argumentsList[0] is undefined, return null.

  2. Let nameOrIndex be the result of converting argumentsList[0] to a DOMString.

  3. Let result be the result of getting the "all"-indexed or named element(s) from this HTMLAllCollection given nameOrIndex.

  4. Return the result of converting result to an ECMAScript value.

The thisArgument is ignored, and thus code such as Function.prototype.call.call(document.all, null, "x") will still search for elements. (document.all.call does not exist, since document.all does not inherit from Function.prototype.)

2.6.3.2 The HTMLFormControlsCollection interface

The HTMLFormControlsCollection interface is used for collections of listed elements in form elements.

HTMLFormControlsCollection

Support in all current engines.

Firefox1+Safari4+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet ExplorerNo
Firefox Android?Safari iOS1+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

RadioNodeList

Support in all current engines.

Firefox33+Safari7+Chrome21+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?
[Exposed=Window]
interface HTMLFormControlsCollection : HTMLCollection {
  // inherits length and item()
  getter (RadioNodeList or Element)? namedItem(DOMString name); // shadows inherited namedItem()
};

[Exposed=Window]
interface RadioNodeList : NodeList {
  attribute DOMString value;
};
collection.length

Returns the number of elements in collection.

element = collection.item(index)
element = collection[index]

Returns the item at index index in collection. The items are sorted in tree order.

element = collection.namedItem(name)

HTMLFormControlsCollection/namedItem

Support in all current engines.

Firefox33+Safari4+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet ExplorerNo
Firefox Android?Safari iOS1+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+
radioNodeList = collection.namedItem(name)
element = collection[name]
radioNodeList = collection[name]

Returns the item with ID or name name from collection.

If there are multiple matching items, then a RadioNodeList object containing all those elements is returned.

radioNodeList.value

Returns the value of the first checked radio button represented by radioNodeList.

radioNodeList.value = value

Checks the first radio button represented by radioNodeList that has value value.

The object's supported property indices are as defined for HTMLCollection objects.

The supported property names consist of the non-empty values of all the id and name attributes of all the elements represented by the collection, in tree order, ignoring later duplicates, with the id of an element preceding its name if it contributes both, they differ from each other, and neither is the duplicate of an earlier entry.

The namedItem(name) method must act according to the following algorithm:

  1. If name is the empty string, return null and stop the algorithm.
  2. If, at the time the method is called, there is exactly one node in the collection that has either an id attribute or a name attribute equal to name, then return that node and stop the algorithm.
  3. Otherwise, if there are no nodes in the collection that have either an id attribute or a name attribute equal to name, then return null and stop the algorithm.
  4. Otherwise, create a new RadioNodeList object representing a live view of the HTMLFormControlsCollection object, further filtered so that the only nodes in the RadioNodeList object are those that have either an id attribute or a name attribute equal to name. The nodes in the RadioNodeList object must be sorted in tree order.
  5. Return that RadioNodeList object.

Members of the RadioNodeList interface inherited from the NodeList interface must behave as they would on a NodeList object.

RadioNodeList/value

Support in all current engines.

Firefox33+Safari7+Chrome21+
Opera?Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

The value IDL attribute on the RadioNodeList object, on getting, must return the value returned by running the following steps:

  1. Let element be the first element in tree order represented by the RadioNodeList object that is an input element whose type attribute is in the Radio Button state and whose checkedness is true. Otherwise, let it be null.

  2. If element is null, return the empty string.

  3. If element is an element with no value attribute, return the string "on".

  4. Otherwise, return the value of element's value attribute.

On setting, the value IDL attribute must run the following steps:

  1. If the new value is the string "on": let element be the first element in tree order represented by the RadioNodeList object that is an input element whose type attribute is in the Radio Button state and whose value content attribute is either absent, or present and equal to the new value, if any. If no such element exists, then instead let element be null.

    Otherwise: let element be the first element in tree order represented by the RadioNodeList object that is an input element whose type attribute is in the Radio Button state and whose value content attribute is present and equal to the new value, if any. If no such element exists, then instead let element be null.

  2. If element is not null, then set its checkedness to true.

2.6.3.3 The HTMLOptionsCollection interface

HTMLOptionsCollection

Support in all current engines.

Firefox1+Safari3+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer6+
Firefox Android?Safari iOS1+Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+

The HTMLOptionsCollection interface is used for collections of option elements. It is always rooted on a select element and has attributes and methods that manipulate that element's descendants.

[Exposed=Window]
interface HTMLOptionsCollection : HTMLCollection {
  // inherits item(), namedItem()
  [CEReactions] attribute unsigned long length; // shadows inherited length
  [CEReactions] setter undefined (unsigned long index, HTMLOptionElement? option);
  [CEReactions] undefined add((HTMLOptionElement or HTMLOptGroupElement) element, optional (HTMLElement or long)? before = null);
  [CEReactions] undefined remove(long index);
  attribute long selectedIndex;
};
collection.length

Returns the number of elements in collection.

collection.length = value

When set to a smaller number than the existing length, truncates the number of option elements in the container corresponding to collection.

When set to a greater number than the existing length, if that number is less than or equal to 100000, adds new blank option elements to the container corresponding to collection.

element = collection.item(index)
element = collection[index]

Returns the item at index index in collection. The items are sorted in tree order.

collection[index] = element

When index is a greater number than the number of items in collection, adds new blank option elements in the corresponding container.

When set to null, removes the item at index index from collection.

When set to an option element, adds or replaces it at index index in collection.

element = collection.namedItem(name)
element = collection[name]

Returns the item with ID or name name from collection.

If there are multiple matching items, then the first is returned.

collection.add(element[, before])

Inserts element before the node given by before.

The before argument can be a number, in which case element is inserted before the item with that number, or an element from collection, in which case element is inserted before that element.

If before is omitted, null, or a number out of range, then element will be added at the end of the list.

Throws a "HierarchyRequestError" DOMException if element is an ancestor of the element into which it is to be inserted.

collection.remove(index)

Removes the item with index index from collection.

collection.selectedIndex

Returns the index of the first selected item, if any, or −1 if there is no selected item.

collection.selectedIndex = index

Changes the selection to the option element at index index in collection.

The object's supported property indices are as defined for HTMLCollection objects.

The length getter steps are to return the number of nodes represented by the collection.

The length setter steps are:

  1. Let current be the number of nodes represented by the collection.

  2. If the given value is greater than current, then:

    1. If the given value is greater than 100,000, then return.

    2. Let n be valuecurrent.

    3. Append n new option elements with no attributes and no child nodes to the select element on which this is rooted.

  3. If the given value is less than current, then:

    1. Let n be currentvalue.

    2. Remove the last n nodes in the collection from their parent nodes.

Setting length never removes or adds any optgroup elements, and never adds new children to existing optgroup elements (though it can remove children from them).

The supported property names consist of the non-empty values of all the id and name attributes of all the elements represented by the collection, in tree order, ignoring later duplicates, with the id of an element preceding its name if it contributes both, they differ from each other, and neither is the duplicate of an earlier entry.

When the user agent is to set the value of a new indexed property or set the value of an existing indexed property for a given property index index to a new value value, it must run the following algorithm:

  1. If value is null, invoke the steps for the remove method with index as the argument, and return.

  2. Let length be the number of nodes represented by the collection.

  3. Let n be index minus length.

  4. If n is greater than zero, then append a DocumentFragment consisting of n-1 new option elements with no attributes and no child nodes to the select element on which the HTMLOptionsCollection is rooted.

  5. If n is greater than or equal to zero, append value to the select element. Otherwise, replace the indexth element in the collection by value.

The add(element, before) method must act according to the following algorithm:

  1. If element is an ancestor of the select element on which the HTMLOptionsCollection is rooted, then throw a "HierarchyRequestError" DOMException.

  2. If before is an element, but that element isn't a descendant of the select element on which the HTMLOptionsCollection is rooted, then throw a "NotFoundError" DOMException.

  3. If element and before are the same element, then return.

  4. If before is a node, then let reference be that node. Otherwise, if before is an integer, and there is a beforeth node in the collection, let reference be that node. Otherwise, let reference be null.

  5. If reference is not null, let parent be the parent node of reference. Otherwise, let parent be the select element on which the HTMLOptionsCollection is rooted.

  6. Pre-insert element into parent node before reference.

The remove(index) method must act according to the following algorithm:

  1. If the number of nodes represented by the collection is zero, return.

  2. If index is not a number greater than or equal to 0 and less than the number of nodes represented by the collection, return.

  3. Let element be the indexth element in the collection.

  4. Remove element from its parent node.

The selectedIndex IDL attribute must act like the identically named attribute on the select element on which the HTMLOptionsCollection is rooted

2.6.4 The DOMStringList interface

DOMStringList

Support in all current engines.

Firefox1+Safari5.1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android?Safari iOS?Chrome Android?WebView Android3+Samsung Internet?Opera Android12.1+

The DOMStringList interface is a non-fashionable retro way of representing a list of strings.

[Exposed=(Window,Worker)]
interface DOMStringList {
  readonly attribute unsigned long length;
  getter DOMString? item(unsigned long index);
  boolean contains(DOMString string);
};

New APIs must use sequence<DOMString> or equivalent rather than DOMStringList.

strings.length

Returns the number of strings in strings.

strings[index]
strings.item(index)

Returns the string with index index from strings.

strings.contains(string)

Returns true if strings contains string, and false otherwise.

Each DOMStringList object has an associated list.

The DOMStringList interface supports indexed properties. The supported property indices are the indices of this's associated list.

DOMStringList/length

Support in all current engines.

Firefox1+Safari5.1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android12.1+

The length getter steps are to return this's associated list's size.

DOMStringList/item

Support in all current engines.

Firefox1+Safari5.1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android?Safari iOS?Chrome Android?WebView Android3+Samsung Internet?Opera Android12.1+

The item(index) method steps are to return the indexth item in this's associated list, or null if index plus one is greater than this's associated list's size.

DOMStringList/contains

Support in all current engines.

Firefox1.5+Safari5.1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer10+
Firefox Android?Safari iOS?Chrome Android?WebView Android3+Samsung Internet?Opera Android12.1+

The contains(string) method steps are to return true if this's associated list contains string, and false otherwise.

2.7 Safe passing of structured data

To support passing JavaScript objects, including platform objects, across realm boundaries, this specification defines the following infrastructure for serializing and deserializing objects, including in some cases transferring the underlying data instead of copying it. Collectively this serialization/deserialization process is known as "structured cloning", although most APIs perform separate serialization and deserialization steps. (With the notable exception being the structuredClone() method.)

This section uses the terminology and typographic conventions from the JavaScript specification. [JAVASCRIPT]

2.7.1 Serializable objects

/developer.mozilla.org/en-US/docs/Glossary/Serializable_object

Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

/developer.mozilla.org/en-US/docs/Glossary/Serializable_object

Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

/developer.mozilla.org/en-US/docs/Glossary/Serializable_object

Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

/developer.mozilla.org/en-US/docs/Glossary/Serializable_object

Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

/developer.mozilla.org/en-US/docs/Glossary/Serializable_object

Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

/developer.mozilla.org/en-US/docs/Glossary/Serializable_object

Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

/developer.mozilla.org/en-US/docs/Glossary/Serializable_object

Firefox103+SafariNoChrome77+
Opera?Edge79+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

Serializable objects support being serialized, and later deserialized, in a way that is independent of any given realm. This allows them to be stored on disk and later restored, or cloned across agent and even agent cluster boundaries.

Not all objects are serializable objects, and not all aspects of objects that are serializable objects are necessarily preserved when they are serialized.

Platform objects can be serializable objects if their primary interface is decorated with the [Serializable] IDL extended attribute. Such interfaces must also define the following algorithms:

serialization steps, taking a platform object value, a Record serialized, and a boolean forStorage

A set of steps that serializes the data in value into fields of serialized. The resulting data serialized into serialized must be independent of any realm.

These steps may throw an exception if serialization is not possible.

These steps may perform a sub-serialization to serialize nested data structures. They should not call StructuredSerialize directly, as doing so will omit the important memory argument.

The introduction of these steps should omit mention of the forStorage argument if it is not relevant to the algorithm.

deserialization steps, taking a Record serialized, a platform object value, and a realm targetRealm

A set of steps that deserializes the data in serialized, using it to set up value as appropriate. value will be a newly-created instance of the platform object type in question, with none of its internal data set up; setting that up is the job of these steps.

These steps may throw an exception if deserialization is not possible.

These steps may perform a sub-deserialization to deserialize nested data structures. They should not call StructuredDeserialize directly, as doing so will omit the important targetRealm and memory arguments.

It is up to the definition of individual platform objects to determine what data is serialized and deserialized by these steps. Typically the steps are very symmetric.

The [Serializable] extended attribute must take no arguments, and must only appear on an interface. It must not appear more than once on an interface.

For a given platform object, only the object's primary interface is considered during the (de)serialization process. Thus, if inheritance is involved in defining the interface, each [Serializable]-annotated interface in the inheritance chain needs to define standalone serialization steps and deserialization steps, including taking into account any important data that might come from inherited interfaces.

Let's say we were defining a platform object Person, which had associated with it two pieces of associated data:

We could then define Person instances to be serializable objects by annotating the Person interface with the [Serializable] extended attribute, and defining the following accompanying algorithms:

serialization steps
  1. Set serialized.[[Name]] to value's associated name value.

  2. Let serializedBestFriend be the sub-serialization of value's associated best friend value.

  3. Set serialized.[[BestFriend]] to serializedBestFriend.

deserialization steps
  1. Set value's associated name value to serialized.[[Name]].

  2. Let deserializedBestFriend be the sub-deserialization of serialized.[[BestFriend]].

  3. Set value's associated best friend value to deserializedBestFriend.

Objects defined in the JavaScript specification are handled by the StructuredSerialize abstract operation directly.

Originally, this specification defined the concept of "cloneable objects", which could be cloned from one realm to another. However, to better specify the behavior of certain more complex situations, the model was updated to make the serialization and deserialization explicit.

2.7.2 Transferable objects

Transferable objects support being transferred across agents. Transferring is effectively recreating the object while sharing a reference to the underlying data and then detaching the object being transferred. This is useful to transfer ownership of expensive resources. Not all objects are transferable objects and not all aspects of objects that are transferable objects are necessarily preserved when transferred.

Transferring is an irreversible and non-idempotent operation. Once an object has been transferred, it cannot be transferred, or indeed used, again.

Platform objects can be transferable objects if their primary interface is decorated with the [Transferable] IDL extended attribute. Such interfaces must also define the following algorithms:

transfer steps, taking a platform object value and a Record dataHolder

A set of steps that transfers the data in value into fields of dataHolder. The resulting data held in dataHolder must be independent of any realm.

These steps may throw an exception if transferral is not possible.

transfer-receiving steps, taking a Record dataHolder and a platform object value

A set of steps that receives the data in dataHolder, using it to set up value as appropriate. value will be a newly-created instance of the platform object type in question, with none of its internal data set up; setting that up is the job of these steps.

These steps may throw an exception if it is not possible to receive the transfer.

It is up to the definition of individual platform objects to determine what data is transferred by these steps. Typically the steps are very symmetric.

The [Transferable] extended attribute must take no arguments, and must only appear on an interface. It must not appear more than once on an interface.

For a given platform object, only the object's primary interface is considered during the transferring process. Thus, if inheritance is involved in defining the interface, each [Transferable]-annotated interface in the inheritance chain needs to define standalone transfer steps and transfer-receiving steps, including taking into account any important data that might come from inherited interfaces.

Platform objects that are transferable objects have a [[Detached]] internal slot. This is used to ensure that once a platform object has been transferred, it cannot be transferred again.

Objects defined in the JavaScript specification are handled by the StructuredSerializeWithTransfer abstract operation directly.

2.7.3 StructuredSerializeInternal ( value, forStorage [ , memory ] )

The StructuredSerializeInternal abstract operation takes as input a JavaScript value value and serializes it to a realm-independent form, represented here as a Record. This serialized form has all the information necessary to later deserialize into a new JavaScript value in a different realm.

This process can throw an exception, for example when trying to serialize un-serializable objects.

  1. If memory was not supplied, let memory be an empty map.

    The purpose of the memory map is to avoid serializing objects twice. This ends up preserving cycles and the identity of duplicate objects in graphs.

  2. If memory[value] exists, then return memory[value].

  3. Let deep be false.

  4. If value is undefined, null, a Boolean, a Number, a BigInt, or a String, then return { [[Type]]: "primitive", [[Value]]: value }.

  5. If value is a Symbol, then throw a "DataCloneError" DOMException.

  6. Let serialized be an uninitialized value.

  7. If value has a [[BooleanData]] internal slot, then set serialized to { [[Type]]: "Boolean", [[BooleanData]]: value.[[BooleanData]] }.

  8. Otherwise, if value has a [[NumberData]] internal slot, then set serialized to { [[Type]]: "Number", [[NumberData]]: value.[[NumberData]] }.

  9. Otherwise, if value has a [[BigIntData]] internal slot, then set serialized to { [[Type]]: "BigInt", [[BigIntData]]: value.[[BigIntData]] }.

  10. Otherwise, if value has a [[StringData]] internal slot, then set serialized to { [[Type]]: "String", [[StringData]]: value.[[StringData]] }.

  11. Otherwise, if value has a [[DateValue]] internal slot, then set serialized to { [[Type]]: "Date", [[DateValue]]: value.[[DateValue]] }.

  12. Otherwise, if value has a [[RegExpMatcher]] internal slot, then set serialized to { [[Type]]: "RegExp", [[RegExpMatcher]]: value.[[RegExpMatcher]], [[OriginalSource]]: value.[[OriginalSource]], [[OriginalFlags]]: value.[[OriginalFlags]] }.

  13. Otherwise, if value has an [[ArrayBufferData]] internal slot, then:

    1. If IsSharedArrayBuffer(value) is true, then:

      1. If the current settings object's cross-origin isolated capability is false, then throw a "DataCloneError" DOMException.

        This check is only needed when serializing (and not when deserializing) as the cross-origin isolated capability cannot change over time and a SharedArrayBuffer cannot leave an agent cluster.

      2. If forStorage is true, then throw a "DataCloneError" DOMException.

      3. If value has an [[ArrayBufferMaxByteLength]] internal slot, then set serialized to { [[Type]]: "GrowableSharedArrayBuffer", [[ArrayBufferData]]: value.[[ArrayBufferData]], [[ArrayBufferByteLengthData]]: value.[[ArrayBufferByteLengthData]], [[ArrayBufferMaxByteLength]]: value.[[ArrayBufferMaxByteLength]], [[AgentCluster]]: the surrounding agent's agent cluster }.

      4. Otherwise, set serialized to { [[Type]]: "SharedArrayBuffer", [[ArrayBufferData]]: value.[[ArrayBufferData]], [[ArrayBufferByteLength]]: value.[[ArrayBufferByteLength]], [[AgentCluster]]: the surrounding agent's agent cluster }.

    2. Otherwise:

      1. If IsDetachedBuffer(value) is true, then throw a "DataCloneError" DOMException.

      2. Let size be value.[[ArrayBufferByteLength]].

      3. Let dataCopy be ? CreateByteDataBlock(size).

        This can throw a RangeError exception upon allocation failure.

      4. Perform CopyDataBlockBytes(dataCopy, 0, value.[[ArrayBufferData]], 0, size).

      5. If value has an [[ArrayBufferMaxByteLength]] internal slot, then set serialized to { [[Type]]: "ResizableArrayBuffer", [[ArrayBufferData]]: dataCopy, [[ArrayBufferByteLength]]: size, [[ArrayBufferMaxByteLength]]: value.[[ArrayBufferMaxByteLength]] }.

      6. Otherwise, set serialized to { [[Type]]: "ArrayBuffer", [[ArrayBufferData]]: dataCopy, [[ArrayBufferByteLength]]: size }.

  14. Otherwise, if value has a [[ViewedArrayBuffer]] internal slot, then:

    1. If IsArrayBufferViewOutOfBounds(value) is true, then throw a "DataCloneError" DOMException.

    2. Let buffer be the value of value's [[ViewedArrayBuffer]] internal slot.

    3. Let bufferSerialized be ? StructuredSerializeInternal(buffer, forStorage, memory).

    4. Assert: bufferSerialized.[[Type]] is "ArrayBuffer", "ResizableArrayBuffer", "SharedArrayBuffer", or "GrowableSharedArrayBuffer".

    5. If value has a [[DataView]] internal slot, then set serialized to { [[Type]]: "ArrayBufferView", [[Constructor]]: "DataView", [[ArrayBufferSerialized]]: bufferSerialized, [[ByteLength]]: value.[[ByteLength]], [[ByteOffset]]: value.[[ByteOffset]] }.

    6. Otherwise:

      1. Assert: value has a [[TypedArrayName]] internal slot.

      2. Set serialized to { [[Type]]: "ArrayBufferView", [[Constructor]]: value.[[TypedArrayName]], [[ArrayBufferSerialized]]: bufferSerialized, [[ByteLength]]: value.[[ByteLength]], [[ByteOffset]]: value.[[ByteOffset]], [[ArrayLength]]: value.[[ArrayLength]] }.

  15. Otherwise, if value has [[MapData]] internal slot, then:

    1. Set serialized to { [[Type]]: "Map", [[MapData]]: a new empty List }.

    2. Set deep to true.

  16. Otherwise, if value has [[SetData]] internal slot, then:

    1. Set serialized to { [[Type]]: "Set", [[SetData]]: a new empty List }.

    2. Set deep to true.

  17. Otherwise, if value has an [[ErrorData]] internal slot and value is not a platform object, then:

    1. Let name be ? Get(value, "name").

    2. If name is not one of "Error", "EvalError", "RangeError", "ReferenceError", "SyntaxError", "TypeError", or "URIError", then set name to "Error".

    3. Let valueMessageDesc be ? value.[[GetOwnProperty]]("message").

    4. Let message be undefined if IsDataDescriptor(valueMessageDesc) is false, and ? ToString(valueMessageDesc.[[Value]]) otherwise.

    5. Set serialized to { [[Type]]: "Error", [[Name]]: name, [[Message]]: message }.

    6. User agents should attach a serialized representation of any interesting accompanying data which are not yet specified, notably the stack property, to serialized.

      See the Error Stacks proposal for in-progress work on specifying this data. [JSERRORSTACKS]

  18. Otherwise, if value is an Array exotic object, then:

    1. Let valueLenDescriptor be ? OrdinaryGetOwnProperty(value, "length").

    2. Let valueLen be valueLenDescriptor.[[Value]].

    3. Set serialized to { [[Type]]: "Array", [[Length]]: valueLen, [[Properties]]: a new empty List }.

    4. Set deep to true.

  19. Otherwise, if value is a platform object that is a serializable object:

    1. If value has a [[Detached]] internal slot whose value is true, then throw a "DataCloneError" DOMException.

    2. Let typeString be the identifier of the primary interface of value.

    3. Set serialized to { [[Type]]: typeString }.

    4. Set deep to true.

  20. Otherwise, if value is a platform object, then throw a "DataCloneError" DOMException.

  21. Otherwise, if IsCallable(value) is true, then throw a "DataCloneError" DOMException.

  22. Otherwise, if value has any internal slot other than [[Prototype]], [[Extensible]], or [[PrivateElements]], then throw a "DataCloneError" DOMException.

    For instance, a [[PromiseState]] or [[WeakMapData]] internal slot.

  23. Otherwise, if value is an exotic object and value is not the %Object.prototype% intrinsic object associated with any realm, then throw a "DataCloneError" DOMException.

    For instance, a proxy object.

  24. Otherwise:

    1. Set serialized to { [[Type]]: "Object", [[Properties]]: a new empty List }.

    2. Set deep to true.

    %Object.prototype% will end up being handled via this step and subsequent steps. The end result is that its exoticness is ignored, and after deserialization the result will be an empty object (not an immutable prototype exotic object).

  25. Set memory[value] to serialized.

  26. If deep is true, then:

    1. If value has a [[MapData]] internal slot, then:

      1. Let copiedList be a new empty List.

      2. For each Record { [[Key]], [[Value]] } entry of value.[[MapData]]:

        1. Let copiedEntry be a new Record { [[Key]]: entry.[[Key]], [[Value]]: entry.[[Value]] }.

        2. If copiedEntry.[[Key]] is not the special value empty, append copiedEntry to copiedList.

      3. For each Record { [[Key]], [[Value]] } entry of copiedList:

        1. Let serializedKey be ? StructuredSerializeInternal(entry.[[Key]], forStorage, memory).

        2. Let serializedValue be ? StructuredSerializeInternal(entry.[[Value]], forStorage, memory).

        3. Append { [[Key]]: serializedKey, [[Value]]: serializedValue } to serialized.[[MapData]].

    2. Otherwise, if value has a [[SetData]] internal slot, then:

      1. Let copiedList be a new empty List.

      2. For each entry of value.[[SetData]]:

        1. If entry is not the special value empty, append entry to copiedList.

      3. For each entry of copiedList:

        1. Let serializedEntry be ? StructuredSerializeInternal(entry, forStorage, memory).

        2. Append serializedEntry to serialized.[[SetData]].

    3. Otherwise, if value is a platform object that is a serializable object, then perform the serialization steps for value's primary interface, given value, serialized, and forStorage.

      The serialization steps may need to perform a sub-serialization. This is an operation which takes as input a value subValue, and returns StructuredSerializeInternal(subValue, forStorage, memory). (In other words, a sub-serialization is a specialization of StructuredSerializeInternal to be consistent within this invocation.)

    4. Otherwise, for each key in ! EnumerableOwnProperties(value, key):

      1. If ! HasOwnProperty(value, key) is true, then:

        1. Let inputValue be ? value.[[Get]](key, value).

        2. Let outputValue be ? StructuredSerializeInternal(inputValue, forStorage, memory).

        3. Append { [[Key]]: key, [[Value]]: outputValue } to serialized.[[Properties]].

  27. Return serialized.

It's important to realize that the Records produced by StructuredSerializeInternal might contain "pointers" to other records that create circular references. For example, when we pass the following JavaScript object into StructuredSerializeInternal:

const o = {};
o.myself = o;

it produces the following result:

{
  [[Type]]: "Object",
  [[Properties]]: «
    {
      [[Key]]: "myself",
      [[Value]]: <a pointer to this whole structure>
    }
  »
}

2.7.4 StructuredSerialize ( value )

  1. Return ? StructuredSerializeInternal(value, false).

2.7.5 StructuredSerializeForStorage ( value )

  1. Return ? StructuredSerializeInternal(value, true).

2.7.6 StructuredDeserialize ( serialized, targetRealm [ , memory ] )

The StructuredDeserialize abstract operation takes as input a Record serialized, which was previously produced by StructuredSerialize or StructuredSerializeForStorage, and deserializes it into a new JavaScript value, created in targetRealm.

This process can throw an exception, for example when trying to allocate memory for the new objects (especially ArrayBuffer objects).

  1. If memory was not supplied, let memory be an empty map.

    The purpose of the memory map is to avoid deserializing objects twice. This ends up preserving cycles and the identity of duplicate objects in graphs.

  2. If memory[serialized] exists, then return memory[serialized].

  3. Let deep be false.

  4. Let value be an uninitialized value.

  5. If serialized.[[Type]] is "primitive", then set value to serialized.[[Value]].

  6. Otherwise, if serialized.[[Type]] is "Boolean", then set value to a new Boolean object in targetRealm whose [[BooleanData]] internal slot value is serialized.[[BooleanData]].

  7. Otherwise, if serialized.[[Type]] is "Number", then set value to a new Number object in targetRealm whose [[NumberData]] internal slot value is serialized.[[NumberData]].

  8. Otherwise, if serialized.[[Type]] is "BigInt", then set value to a new BigInt object in targetRealm whose [[BigIntData]] internal slot value is serialized.[[BigIntData]].

  9. Otherwise, if serialized.[[Type]] is "String", then set value to a new String object in targetRealm whose [[StringData]] internal slot value is serialized.[[StringData]].

  10. Otherwise, if serialized.[[Type]] is "Date", then set value to a new Date object in targetRealm whose [[DateValue]] internal slot value is serialized.[[DateValue]].

  11. Otherwise, if serialized.[[Type]] is "RegExp", then set value to a new RegExp object in targetRealm whose [[RegExpMatcher]] internal slot value is serialized.[[RegExpMatcher]], whose [[OriginalSource]] internal slot value is serialized.[[OriginalSource]], and whose [[OriginalFlags]] internal slot value is serialized.[[OriginalFlags]].

  12. Otherwise, if serialized.[[Type]] is "SharedArrayBuffer", then:

    1. If targetRealm's corresponding agent cluster is not serialized.[[AgentCluster]], then throw a "DataCloneError" DOMException.

    2. Otherwise, set value to a new SharedArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value is serialized.[[ArrayBufferData]] and whose [[ArrayBufferByteLength]] internal slot value is serialized.[[ArrayBufferByteLength]].

  13. Otherwise, if serialized.[[Type]] is "GrowableSharedArrayBuffer", then:

    1. If targetRealm's corresponding agent cluster is not serialized.[[AgentCluster]], then throw a "DataCloneError" DOMException.

    2. Otherwise, set value to a new SharedArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value is serialized.[[ArrayBufferData]], whose [[ArrayBufferByteLengthData]] internal slot value is serialized.[[ArrayBufferByteLengthData]], and whose [[ArrayBufferMaxByteLength]] internal slot value is serialized.[[ArrayBufferMaxByteLength]].

  14. Otherwise, if serialized.[[Type]] is "ArrayBuffer", then set value to a new ArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value is serialized.[[ArrayBufferData]], and whose [[ArrayBufferByteLength]] internal slot value is serialized.[[ArrayBufferByteLength]].

    If this throws an exception, catch it, and then throw a "DataCloneError" DOMException.

    This step might throw an exception if there is not enough memory available to create such an ArrayBuffer object.

  15. Otherwise, if serialized.[[Type]] is "ResizableArrayBuffer", then set value to a new ArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value is serialized.[[ArrayBufferData]], whose [[ArrayBufferByteLength]] internal slot value is serialized.[[ArrayBufferByteLength]], and whose [[ArrayBufferMaxByteLength]] internal slot value is serialized.[[ArrayBufferMaxByteLength]].

    If this throws an exception, catch it, and then throw a "DataCloneError" DOMException.

    This step might throw an exception if there is not enough memory available to create such an ArrayBuffer object.

  16. Otherwise, if serialized.[[Type]] is "ArrayBufferView", then:

    1. Let deserializedArrayBuffer be ? StructuredDeserialize(serialized.[[ArrayBufferSerialized]], targetRealm, memory).

    2. If serialized.[[Constructor]] is "DataView", then set value to a new DataView object in targetRealm whose [[ViewedArrayBuffer]] internal slot value is deserializedArrayBuffer, whose [[ByteLength]] internal slot value is serialized.[[ByteLength]], and whose [[ByteOffset]] internal slot value is serialized.[[ByteOffset]].

    3. Otherwise, set value to a new typed array object in targetRealm, using the constructor given by serialized.[[Constructor]], whose [[ViewedArrayBuffer]] internal slot value is deserializedArrayBuffer, whose [[TypedArrayName]] internal slot value is serialized.[[Constructor]], whose [[ByteLength]] internal slot value is serialized.[[ByteLength]], whose [[ByteOffset]] internal slot value is serialized.[[ByteOffset]], and whose [[ArrayLength]] internal slot value is serialized.[[ArrayLength]].

  17. Otherwise, if serialized.[[Type]] is "Map", then:

    1. Set value to a new Map object in targetRealm whose [[MapData]] internal slot value is a new empty List.

    2. Set deep to true.

  18. Otherwise, if serialized.[[Type]] is "Set", then:

    1. Set value to a new Set object in targetRealm whose [[SetData]] internal slot value is a new empty List.

    2. Set deep to true.

  19. Otherwise, if serialized.[[Type]] is "Array", then:

    1. Let outputProto be targetRealm.[[Intrinsics]].[[%Array.prototype%]].

    2. Set value to ! ArrayCreate(serialized.[[Length]], outputProto).

    3. Set deep to true.

  20. Otherwise, if serialized.[[Type]] is "Object", then:

    1. Set value to a new Object in targetRealm.

    2. Set deep to true.

  21. Otherwise, if serialized.[[Type]] is "Error", then:

    1. Let prototype be %Error.prototype%.

    2. If serialized.[[Name]] is "EvalError", then set prototype to %EvalError.prototype%.

    3. If serialized.[[Name]] is "RangeError", then set prototype to %RangeError.prototype%.

    4. If serialized.[[Name]] is "ReferenceError", then set prototype to %ReferenceError.prototype%.

    5. If serialized.[[Name]] is "SyntaxError", then set prototype to %SyntaxError.prototype%.

    6. If serialized.[[Name]] is "TypeError", then set prototype to %TypeError.prototype%.

    7. If serialized.[[Name]] is "URIError", then set prototype to %URIError.prototype%.

    8. Let message be serialized.[[Message]].

    9. Set value to OrdinaryObjectCreate(prototype, « [[ErrorData]] »).

    10. Let messageDesc be PropertyDescriptor{ [[Value]]: message, [[Writable]]: true, [[Enumerable]]: false, [[Configurable]]: true }.

    11. If message is not undefined, then perform ! OrdinaryDefineOwnProperty(value, "message", messageDesc).

    12. Any interesting accompanying data attached to serialized should be deserialized and attached to value.

  22. Otherwise:

    1. Let interfaceName be serialized.[[Type]].

    2. If the interface identified by interfaceName is not exposed in targetRealm, then throw a "DataCloneError" DOMException.

    3. Set value to a new instance of the interface identified by interfaceName, created in targetRealm.

    4. Set deep to true.

  23. Set memory[serialized] to value.

  24. If deep is true, then:

    1. If serialized.[[Type]] is "Map", then:

      1. For each Record { [[Key]], [[Value]] } entry of serialized.[[MapData]]:

        1. Let deserializedKey be ? StructuredDeserialize(entry.[[Key]], targetRealm, memory).

        2. Let deserializedValue be ? StructuredDeserialize(entry.[[Value]], targetRealm, memory).

        3. Append { [[Key]]: deserializedKey, [[Value]]: deserializedValue } to value.[[MapData]].

    2. Otherwise, if serialized.[[Type]] is "Set", then:

      1. For each entry of serialized.[[SetData]]:

        1. Let deserializedEntry be ? StructuredDeserialize(entry, targetRealm, memory).

        2. Append deserializedEntry to value.[[SetData]].

    3. Otherwise, if serialized.[[Type]] is "Array" or "Object", then:

      1. For each Record { [[Key]], [[Value]] } entry of serialized.[[Properties]]:

        1. Let deserializedValue be ? StructuredDeserialize(entry.[[Value]], targetRealm, memory).

        2. Let result be ! CreateDataProperty(value, entry.[[Key]], deserializedValue).

        3. Assert: result is true.

    4. Otherwise:

      1. Perform the appropriate deserialization steps for the interface identified by serialized.[[Type]], given serialized, value, and targetRealm.

        The deserialization steps may need to perform a sub-deserialization. This is an operation which takes as input a previously-serialized Record subSerialized, and returns StructuredDeserialize(subSerialized, targetRealm, memory). (In other words, a sub-deserialization is a specialization of StructuredDeserialize to be consistent within this invocation.)

  25. Return value.

2.7.7 StructuredSerializeWithTransfer ( value, transferList )

  1. Let memory be an empty map.

    In addition to how it is used normally by StructuredSerializeInternal, in this algorithm memory is also used to ensure that StructuredSerializeInternal ignores items in transferList, and let us do our own handling instead.

  2. For each transferable of transferList:

    1. If transferable has neither an [[ArrayBufferData]] internal slot nor a [[Detached]] internal slot, then throw a "DataCloneError" DOMException.

    2. If transferable has an [[ArrayBufferData]] internal slot and IsSharedArrayBuffer(transferable) is true, then throw a "DataCloneError" DOMException.

    3. If memory[transferable] exists, then throw a "DataCloneError" DOMException.

    4. Set memory[transferable] to { [[Type]]: an uninitialized value }.

      transferable is not transferred yet as transferring has side effects and StructuredSerializeInternal needs to be able to throw first.

  3. Let serialized be ? StructuredSerializeInternal(value, false, memory).

  4. Let transferDataHolders be a new empty List.

  5. For each transferable of transferList:

    1. If transferable has an [[ArrayBufferData]] internal slot and IsDetachedBuffer(transferable) is true, then throw a "DataCloneError" DOMException.

    2. If transferable has a [[Detached]] internal slot and transferable.[[Detached]] is true, then throw a "DataCloneError" DOMException.

    3. Let dataHolder be memory[transferable].

    4. If transferable has an [[ArrayBufferData]] internal slot, then:

      1. If transferable has an [[ArrayBufferMaxByteLength]] internal slot, then:

        1. Set dataHolder.[[Type]] to "ResizableArrayBuffer".

        2. Set dataHolder.[[ArrayBufferData]] to transferable.[[ArrayBufferData]].

        3. Set dataHolder.[[ArrayBufferByteLength]] to transferable.[[ArrayBufferByteLength]].

        4. Set dataHolder.[[ArrayBufferMaxByteLength]] to transferable.[[ArrayBufferMaxByteLength]].

      2. Otherwise:

        1. Set dataHolder.[[Type]] to "ArrayBuffer".

        2. Set dataHolder.[[ArrayBufferData]] to transferable.[[ArrayBufferData]].

        3. Set dataHolder.[[ArrayBufferByteLength]] to transferable.[[ArrayBufferByteLength]].

      3. Perform ? DetachArrayBuffer(transferable).

        Specifications can use the [[ArrayBufferDetachKey]] internal slot to prevent ArrayBuffers from being detached. This is used in WebAssembly JavaScript Interface, for example. [WASMJS]

    5. Otherwise:

      1. Assert: transferable is a platform object that is a transferable object.

      2. Let interfaceName be the identifier of the primary interface of transferable.

      3. Set dataHolder.[[Type]] to interfaceName.

      4. Perform the appropriate transfer steps for the interface identified by interfaceName, given transferable and dataHolder.

      5. Set transferable.[[Detached]] to true.

    6. Append dataHolder to transferDataHolders.

  6. Return { [[Serialized]]: serialized, [[TransferDataHolders]]: transferDataHolders }.

2.7.8 StructuredDeserializeWithTransfer ( serializeWithTransferResult, targetRealm )

  1. Let memory be an empty map.

    Analogous to StructuredSerializeWithTransfer, in addition to how it is used normally by StructuredDeserialize, in this algorithm memory is also used to ensure that StructuredDeserialize ignores items in serializeWithTransferResult.[[TransferDataHolders]], and let us do our own handling instead.

  2. Let transferredValues be a new empty List.

  3. For each transferDataHolder of serializeWithTransferResult.[[TransferDataHolders]]:

    1. Let value be an uninitialized value.

    2. If transferDataHolder.[[Type]] is "ArrayBuffer", then set value to a new ArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value is transferDataHolder.[[ArrayBufferData]], and whose [[ArrayBufferByteLength]] internal slot value is transferDataHolder.[[ArrayBufferByteLength]].

      In cases where the original memory occupied by [[ArrayBufferData]] is accessible during the deserialization, this step is unlikely to throw an exception, as no new memory needs to be allocated: the memory occupied by [[ArrayBufferData]] is instead just getting transferred into the new ArrayBuffer. This could be true, for example, when both the source and target realms are in the same process.

    3. Otherwise, if transferDataHolder.[[Type]] is "ResizableArrayBuffer", then set value to a new ArrayBuffer object in targetRealm whose [[ArrayBufferData]] internal slot value is transferDataHolder.[[ArrayBufferData]], whose [[ArrayBufferByteLength]] internal slot value is transferDataHolder.[[ArrayBufferByteLength]], and whose [[ArrayBufferMaxByteLength]] internal slot value is transferDataHolder.[[ArrayBufferMaxByteLength]].

      For the same reason as the previous step, this step is also unlikely to throw an exception.

    4. Otherwise:

      1. Let interfaceName be transferDataHolder.[[Type]].

      2. If the interface identified by interfaceName is not exposed in targetRealm, then throw a "DataCloneError" DOMException.

      3. Set value to a new instance of the interface identified by interfaceName, created in targetRealm.

      4. Perform the appropriate transfer-receiving steps for the interface identified by interfaceName given transferDataHolder and value.

    5. Set memory[transferDataHolder] to value.

    6. Append value to transferredValues.

  4. Let deserialized be ? StructuredDeserialize(serializeWithTransferResult.[[Serialized]], targetRealm, memory).

  5. Return { [[Deserialized]]: deserialized, [[TransferredValues]]: transferredValues }.

2.7.9 Performing serialization and transferring from other specifications

Other specifications may use the abstract operations defined here. The following provides some guidance on when each abstract operation is typically useful, with examples.

StructuredSerializeWithTransfer
StructuredDeserializeWithTransfer

Cloning a value to another realm, with a transfer list, but where the target realm is not known ahead of time. In this case the serialization step can be performed immediately, with the deserialization step delayed until the target realm becomes known.

messagePort.postMessage() uses this pair of abstract operations, as the destination realm is not known until the MessagePort has been shipped.

StructuredSerialize
StructuredSerializeForStorage
StructuredDeserialize

Creating a realm-independent snapshot of a given value which can be saved for an indefinite amount of time, and then reified back into a JavaScript value later, possibly multiple times.

StructuredSerializeForStorage can be used for situations where the serialization is anticipated to be stored in a persistent manner, instead of passed between realms. It throws when attempting to serialize SharedArrayBuffer objects, since storing shared memory does not make sense. Similarly, it can throw or possibly have different behavior when given a platform object with custom serialization steps when the forStorage argument is true.

history.pushState() and history.replaceState() use StructuredSerializeForStorage on author-supplied state objects, storing them as serialized state in the appropriate session history entry. Then, StructuredDeserialize is used so that the history.state property can return a clone of the originally-supplied state object.

broadcastChannel.postMessage() uses StructuredSerialize on its input, then uses StructuredDeserialize multiple times on the result to produce a fresh clone for each destination being broadcast to. Note that transferring does not make sense in multi-destination situations.

Any API for persisting JavaScript values to the filesystem would also use StructuredSerializeForStorage on its input and StructuredDeserialize on its output.

In general, call sites may pass in Web IDL values instead of JavaScript values; this is to be understood to perform an implicit conversion to the JavaScript value before invoking these algorithms.


Call sites that are not invoked as a result of author code synchronously calling into a user agent method must take care to properly prepare to run script and prepare to run a callback before invoking StructuredSerialize, StructuredSerializeForStorage, or StructuredSerializeWithTransfer abstract operations, if they are being performed on arbitrary objects. This is necessary because the serialization process can invoke author-defined accessors as part of its final deep-serialization steps, and these accessors could call into operations that rely on the entry and incumbent concepts being properly set up.

window.postMessage() performs StructuredSerializeWithTransfer on its arguments, but is careful to do so immediately, inside the synchronous portion of its algorithm. Thus it is able to use the algorithms without needing to prepare to run script and prepare to run a callback.

In contrast, a hypothetical API that used StructuredSerialize to serialize some author-supplied object periodically, directly from a task on the event loop, would need to ensure it performs the appropriate preparations beforehand. As of this time, we know of no such APIs on the platform; usually it is simpler to perform the serialization ahead of time, as a synchronous consequence of author code.

2.7.10 Structured cloning API

result = self.structuredClone(value[, { transfer }])

Takes the input value and returns a deep copy by performing the structured clone algorithm. Transferable objects listed in the transfer array are transferred, not just cloned, meaning that they are no longer usable in the input value.

Throws a "DataCloneError" DOMException if any part of the input value is not serializable.

structuredClone

Support in all current engines.

Firefox94+Safari15.4+Chrome98+
Opera?Edge98+
Edge (Legacy)?Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

The structuredClone(value, options) method steps are:

  1. Let serialized be ? StructuredSerializeWithTransfer(value, options["transfer"]).

  2. Let deserializeRecord be ? StructuredDeserializeWithTransfer(serialized, this's relevant realm).

  3. Return deserializeRecord.[[Deserialized]].

3 Semantics, structure, and APIs of HTML documents

3.1 Documents

Every XML and HTML document in an HTML UA is represented by a Document object. [DOM]

The Document object's URL is defined in DOM. It is initially set when the Document object is created, but can change during the lifetime of the Document object; for example, it changes when the user navigates to a fragment on the page and when the pushState() method is called with a new URL. [DOM]

Interactive user agents typically expose the Document object's URL in their user interface. This is the primary mechanism by which a user can tell if a site is attempting to impersonate another.

The Document object's origin is defined in DOM. It is initially set when the Document object is created, and can change during the lifetime of the Document only upon setting document.domain. A Document's origin can differ from the origin of its URL; for example when a child navigable is created, its active document's origin is inherited from its parent's active document's origin, even though its active document's URL is about:blank. [DOM]

When a Document is created by a script using the createDocument() or createHTMLDocument() methods, the Document is ready for post-load tasks immediately.

The document's referrer is a string (representing a URL) that can be set when the Document is created. If it is not explicitly set, then its value is the empty string.

3.1.1 The Document object

Document

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera3+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android37+Samsung Internet?Opera Android10.1+

DOM defines a Document interface, which this specification extends significantly.

enum DocumentReadyState { "loading", "interactive", "complete" };
enum DocumentVisibilityState { "visible", "hidden" };
typedef (HTMLScriptElement or SVGScriptElement) HTMLOrSVGScriptElement;

[LegacyOverrideBuiltIns]
partial interface Document {
  static Document parseHTMLUnsafe((TrustedHTML or DOMString) html);

  // resource metadata management
  [PutForwards=href, LegacyUnforgeable] readonly attribute Location? location;
  attribute USVString domain;
  readonly attribute USVString referrer;
  attribute USVString cookie;
  readonly attribute DOMString lastModified;
  readonly attribute DocumentReadyState readyState;

  // DOM tree accessors
  getter object (DOMString name);
  [CEReactions] attribute DOMString title;
  [CEReactions] attribute DOMString dir;
  [CEReactions] attribute HTMLElement? body;
  readonly attribute HTMLHeadElement? head;
  [SameObject] readonly attribute HTMLCollection images;
  [SameObject] readonly attribute HTMLCollection embeds;
  [SameObject] readonly attribute HTMLCollection plugins;
  [SameObject] readonly attribute HTMLCollection links;
  [SameObject] readonly attribute HTMLCollection forms;
  [SameObject] readonly attribute HTMLCollection scripts;
  NodeList getElementsByName(DOMString elementName);
  readonly attribute HTMLOrSVGScriptElement? currentScript; // classic scripts in a document tree only

  // dynamic markup insertion
  [CEReactions] Document open(optional DOMString unused1, optional DOMString unused2); // both arguments are ignored
  WindowProxy? open(USVString url, DOMString name, DOMString features);
  [CEReactions] undefined close();
  [CEReactions] undefined write((TrustedHTML or DOMString)... text);
  [CEReactions] undefined writeln((TrustedHTML or DOMString)... text);

  // user interaction
  readonly attribute WindowProxy? defaultView;
  boolean hasFocus();
  [CEReactions] attribute DOMString designMode;
  [CEReactions] boolean execCommand(DOMString commandId, optional boolean showUI = false, optional DOMString value = "");
  boolean queryCommandEnabled(DOMString commandId);
  boolean queryCommandIndeterm(DOMString commandId);
  boolean queryCommandState(DOMString commandId);
  boolean queryCommandSupported(DOMString commandId);
  DOMString queryCommandValue(DOMString commandId);
  readonly attribute boolean hidden;
  readonly attribute DocumentVisibilityState visibilityState;

  // special event handler IDL attributes that only apply to Document objects
  [LegacyLenientThis] attribute EventHandler onreadystatechange;
  attribute EventHandler onvisibilitychange;

  // also has obsolete members
};
Document includes GlobalEventHandlers;

Each Document has a policy container (a policy container), initially a new policy container, which contains policies which apply to the Document.

Each Document has a permissions policy, which is a permissions policy, which is initially empty.

Each Document has a module map, which is a module map, initially empty.

Each Document has an opener policy, which is an opener policy, initially a new opener policy.

Each Document has an is initial about:blank, which is a boolean, initially false.

Each Document has a during-loading navigation ID for WebDriver BiDi, which is a navigation ID or null, initially null.

As the name indicates, this is used for interfacing with the WebDriver BiDi specification, which needs to be informed about certain occurrences during the early parts of the Document's lifecycle, in a way that ties them to the original navigation ID used when the navigation that created this Document was the ongoing navigation. This eventually gets set back to null, after WebDriver BiDi considers the loading process to be finished. [BIDI]

Each Document has an about base URL, which is a URL or null, initially null.

This is only populated for "about:"-schemed Documents.

Each Document has a bfcache blocking details, which is a set of not restored reason details, initially empty.

3.1.2 The DocumentOrShadowRoot interface

DOM defines the DocumentOrShadowRoot mixin, which this specification extends.

partial interface mixin DocumentOrShadowRoot {
  readonly attribute Element? activeElement;
};

3.1.3 Resource metadata management

document.referrer

Document/referrer

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera3+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android10.1+

Returns the URL of the Document from which the user navigated to this one, unless it was blocked or there was no such document, in which case it returns the empty string.

The noreferrer link type can be used to block the referrer.

The referrer attribute must return the document's referrer.


document.cookie [ = value ]

Returns the HTTP cookies that apply to the Document. If there are no cookies or cookies can't be applied to this resource, the empty string will be returned.

Can be set, to add a new cookie to the element's set of HTTP cookies.

If the contents are sandboxed into an opaque origin (e.g., in an iframe with the sandbox attribute), a "SecurityError" DOMException will be thrown on getting and setting.

Document/cookie

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera3+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android10.1+

The cookie attribute represents the cookies of the resource identified by the document's URL.

A Document object that falls into one of the following conditions is a cookie-averse Document object:

(This is a tracking vector.) On getting, if the document is a cookie-averse Document object, then the user agent must return the empty string. Otherwise, if the Document's origin is an opaque origin, the user agent must throw a "SecurityError" DOMException. Otherwise, the user agent must return the cookie-string for the document's URL for a "non-HTTP" API, decoded using UTF-8 decode without BOM. [COOKIES]

On setting, if the document is a cookie-averse Document object, then the user agent must do nothing. Otherwise, if the Document's origin is an opaque origin, the user agent must throw a "SecurityError" DOMException. Otherwise, the user agent must act as it would when receiving a set-cookie-string for the document's URL via a "non-HTTP" API, consisting of the new value encoded as UTF-8. [COOKIES] [ENCODING]

Since the cookie attribute is accessible across frames, the path restrictions on cookies are only a tool to help manage which cookies are sent to which parts of the site, and are not in any way a security feature.

The cookie attribute's getter and setter synchronously access shared state. Since there is no locking mechanism, other browsing contexts in a multiprocess user agent can modify cookies while scripts are running. A site could, for instance, try to read a cookie, increment its value, then write it back out, using the new value of the cookie as a unique identifier for the session; if the site does this twice in two different browser windows at the same time, it might end up using the same "unique" identifier for both sessions, with potentially disastrous effects.


document.lastModified

Document/lastModified

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

Returns the date of the last modification to the document, as reported by the server, in the form "MM/DD/YYYY hh:mm:ss", in the user's local time zone.

If the last modification date is not known, the current time is returned instead.

The lastModified attribute, on getting, must return the date and time of the Document's source file's last modification, in the user's local time zone, in the following format:

  1. The month component of the date.

  2. A U+002F SOLIDUS character (/).

  3. The day component of the date.

  4. A U+002F SOLIDUS character (/).

  5. The year component of the date.

  6. A U+0020 SPACE character.

  7. The hours component of the time.

  8. A U+003A COLON character (:).

  9. The minutes component of the time.

  10. A U+003A COLON character (:).

  11. The seconds component of the time.

All the numeric components above, other than the year, must be given as two ASCII digits representing the number in base ten, zero-padded if necessary. The year must be given as the shortest possible string of four or more ASCII digits representing the number in base ten, zero-padded if necessary.

The Document's source file's last modification date and time must be derived from relevant features of the networking protocols used, e.g. from the value of the HTTP `Last-Modified` header of the document, or from metadata in the file system for local files. If the last modification date and time are not known, the attribute must return the current date and time in the above format.

3.1.4 Reporting document loading status

document.readyState

Returns "loading" while the Document is loading, "interactive" once it is finished parsing but still loading subresources, and "complete" once it has loaded.

The readystatechange event fires on the Document object when this value changes.

The DOMContentLoaded event fires after the transition to "interactive" but before the transition to "complete", at the point where all subresources apart from async script elements have loaded.

Document/readyState

Support in all current engines.

Firefox3.6+Safari1+Chrome1+
Opera11+Edge79+
Edge (Legacy)12+Internet Explorer11
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android11+

Each Document has a current document readiness, a string, initially "complete".

For Document objects created via the create and initialize a Document object algorithm, this will be immediately reset to "loading" before any script can observe the value of document.readyState. This default applies to other cases such as initial about:blank Documents or Documents without a browsing context.

The readyState getter steps are to return this's current document readiness.

To update the current document readiness for Document document to readinessValue:

  1. If document's current document readiness equals readinessValue, then return.

  2. Set document's current document readiness to readinessValue.

  3. If document is associated with an HTML parser, then:

    1. Let now be the current high resolution time given document's relevant global object.

    2. If readinessValue is "complete", and document's load timing info's DOM complete time is 0, then set document's load timing info's DOM complete time to now.

    3. Otherwise, if readinessValue is "interactive", and document's load timing info's DOM interactive time is 0, then set document's load timing info's DOM interactive time to now.

  4. Fire an event named readystatechange at document.


A Document is said to have an active parser if it is associated with an HTML parser or an XML parser that has not yet been stopped or aborted.


A Document has a document load timing info load timing info.

A Document has a document unload timing info previous document unload timing.

A Document has a boolean was created via cross-origin redirects, initially false.

The document load timing info struct has the following items:

navigation start time (default 0)
A number
DOM interactive time (default 0)
DOM content loaded event start time (default 0)
DOM content loaded event end time (default 0)
DOM complete time (default 0)
load event start time (default 0)
load event end time (default 0)
DOMHighResTimeStamp values

The document unload timing info struct has the following items:

unload event start time (default 0)
unload event end time (default 0)
DOMHighResTimeStamp values

3.1.5 Render-blocking mechanism

Each Document has a render-blocking element set, a set of elements, initially the empty set.

A Document document allows adding render-blocking elements if document's content type is "text/html" and the body element of document is null.

A Document document is render-blocked if both of the following are true:

An element el is render-blocking if el's node document document is render-blocked, and el is in document's render-blocking element set.

To block rendering on an element el:

  1. Let document be el's node document.

  2. If document allows adding render-blocking elements, then append el to document's render-blocking element set.

To unblock rendering on an element el:

  1. Let document be el's node document.

  2. Remove el from document's render-blocking element set.

Whenever a render-blocking element el becomes browsing-context disconnected, or el's blocking attribute's value is changed so that el is no longer potentially render-blocking, then unblock rendering on el.

3.1.6 DOM tree accessors

The html element of a document is its document element, if it's an html element, and null otherwise.


document.head

Document/head

Support in all current engines.

Firefox4+Safari5+Chrome4+
Opera11+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android?Safari iOS4+Chrome Android?WebView Android37+Samsung Internet?Opera Android11+

Returns the head element.

The head element of a document is the first head element that is a child of the html element, if there is one, or null otherwise.

The head attribute, on getting, must return the head element of the document (a head element or null).


document.title [ = value ]

Returns the document's title, as given by the title element for HTML and as given by the SVG title element for SVG.

Can be set, to update the document's title. If there is no appropriate element to update, the new value is ignored.

The title element of a document is the first title element in the document (in tree order), if there is one, or null otherwise.

Document/title

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

The title attribute must, on getting, run the following algorithm:

  1. If the document element is an SVG svg element, then let value be the child text content of the first SVG title element that is a child of the document element.

  2. Otherwise, let value be the child text content of the title element, or the empty string if the title element is null.

  3. Strip and collapse ASCII whitespace in value.

  4. Return value.

On setting, the steps corresponding to the first matching condition in the following list must be run:

If the document element is an SVG svg element
  1. If there is an SVG title element that is a child of the document element, let element be the first such element.

  2. Otherwise:

    1. Let element be the result of creating an element given the document element's node document, "title", and the SVG namespace.

    2. Insert element as the first child of the document element.

  3. String replace all with the given value within element.

If the document element is in the HTML namespace
  1. If the title element is null and the head element is null, then return.

  2. If the title element is non-null, let element be the title element.

  3. Otherwise:

    1. Let element be the result of creating an element given the document element's node document, "title", and the HTML namespace.

    2. Append element to the head element.

  4. String replace all with the given value within element.

Otherwise

Do nothing.


document.body [ = value ]

Document/body

Support in all current engines.

Firefox60+Safari1+Chrome1+
Opera9.6+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android10.1+

Returns the body element.

Can be set, to replace the body element.

If the new value is not a body or frameset element, this will throw a "HierarchyRequestError" DOMException.

The body element of a document is the first of the html element's children that is either a body element or a frameset element, or null if there is no such element.

The body attribute, on getting, must return the body element of the document (either a body element, a frameset element, or null). On setting, the following algorithm must be run:

  1. If the new value is not a body or frameset element, then throw a "HierarchyRequestError" DOMException.
  2. Otherwise, if the new value is the same as the body element, return.
  3. Otherwise, if the body element is not null, then replace the body element with the new value within the body element's parent and return.
  4. Otherwise, if there is no document element, throw a "HierarchyRequestError" DOMException.
  5. Otherwise, the body element is null, but there's a document element. Append the new value to the document element.

The value returned by the body getter is not always the one passed to the setter.

In this example, the setter successfully inserts a body element (though this is non-conforming since SVG does not allow a body as child of SVG svg). However the getter will return null because the document element is not html.

<svg xmlns="http://www.w3.org/2000/svg">
 <script>
  document.body = document.createElementNS("http://www.w3.org/1999/xhtml", "body");
  console.assert(document.body === null);
 </script>
</svg>

document.images

Document/images

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

Returns an HTMLCollection of the img elements in the Document.

document.embeds

Document/embeds

Support in all current engines.

Firefox1+Safari10.1+Chrome64+
Opera51+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android47+
document.plugins

Document/plugins

Support in all current engines.

Firefox1+Safari10.1+Chrome64+
Opera51+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS10.3+Chrome Android?WebView Android?Samsung Internet?Opera Android47+

Returns an HTMLCollection of the embed elements in the Document.

document.links

Document/links

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

Returns an HTMLCollection of the a and area elements in the Document that have href attributes.

document.forms

Document/forms

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

Returns an HTMLCollection of the form elements in the Document.

document.scripts

Document/scripts

Support in all current engines.

Firefox9+Safari3+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet Explorer4+
Firefox Android?Safari iOS1+Chrome Android?WebView Android?Samsung Internet?Opera Android12.1+

Returns an HTMLCollection of the script elements in the Document.

The images attribute must return an HTMLCollection rooted at the Document node, whose filter matches only img elements.

The embeds attribute must return an HTMLCollection rooted at the Document node, whose filter matches only embed elements.

The plugins attribute must return the same object as that returned by the embeds attribute.

The links attribute must return an HTMLCollection rooted at the Document node, whose filter matches only a elements with href attributes and area elements with href attributes.

The forms attribute must return an HTMLCollection rooted at the Document node, whose filter matches only form elements.

The scripts attribute must return an HTMLCollection rooted at the Document node, whose filter matches only script elements.


collection = document.getElementsByName(name)

Document/getElementsByName

Support in all current engines.

Firefox1+Safari1+Chrome1+
Opera5+Edge79+
Edge (Legacy)12+Internet Explorer5+
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android10.1+

Returns a NodeList of elements in the Document that have a name attribute with the value name.

The getElementsByName(elementName) method steps are to return a live NodeList containing all the HTML elements in that document that have a name attribute whose value is identical to the elementName argument, in tree order. When the method is invoked on a Document object again with the same argument, the user agent may return the same as the object returned by the earlier call. In other cases, a new NodeList object must be returned.


document.currentScript

Document/currentScript

Support in all current engines.

Firefox4+Safari8+Chrome29+
Opera?Edge79+
Edge (Legacy)12+Internet ExplorerNo
Firefox Android?Safari iOS?Chrome Android?WebView Android?Samsung Internet?Opera Android?

Returns the script element, or the SVG script element, that is currently executing, as long as the element represents a classic script. In the case of reentrant script execution, returns the one that most recently started executing amongst those that have not yet finished executing.

Returns null if the Document is not currently executing a script or SVG script element (e.g., because the running script is an event handler, or a timeout), or if the currently executing script or SVG script element represents a module script.

The currentScript attribute, on getting, must return the value to which it was most recently set. When the Document is created, the currentScript must be initialized to null.

This API has fallen out of favor in the implementer and standards community, as it globally exposes script or SVG script elements. As such, it is not available in newer contexts, such as when running module scripts or when running scripts in a shadow tree. We are looking into creating a new solution for identifying the running script in such contexts, which does not make it globally available: see issue #1013.


The Document interface supports named properties. The supported property names of a Document object document at any moment consist of the following, in tree order according to the element that contributed them, ignoring later duplicates, and with values from id attributes coming before values from name attributes when the same element contributes both:

To determine the value of a named property name for a Document, the user agent must return the value obtained using the following steps:

  1. Let elements be the list of named elements with the name name that are in a document tree with the Document as their root.

    There will be at least one such element, since the algorithm would otherwise not have been invoked by Web IDL.

  2. If elements has only one element, and that element is an iframe element, and that iframe element's content navigable is not null, then return the active WindowProxy of the element's content navigable.

  3. Otherwise, if elements has only one element, return that element.

  4. Otherwise, return an HTMLCollection rooted at the Document node, whose filter matches only named elements with the name name.

Named elements with the name name, for the purposes of the above algorithm, are those that are either:

An embed or object element is said to be exposed if it has no exposed object ancestor, and, for object elements, is additionally either not showing its fallback content or has no object or embed descendants.


The dir attribute on the Document interface is defined along with the dir content attribute.

3.2 Elements

3.2.1 Semantics

Elements, attributes, and attribute values in HTML are defined (by this specification) to have certain meanings (semantics). For example, the ol element represents an ordered list, and the lang attribute represents the language of the content.

These definitions allow HTML processors, such as web browsers or search engines, to present and use documents and applications in a wide variety of contexts that the author might not have considered.

As a simple example, consider a web page written by an author who only considered desktop computer web browsers:

<!DOCTYPE HTML>
<html lang="en">
 <head>
  <title>My Page</title>
 </head>
 <body>
  <h1>Welcome to my page</h1>
  <p>I like cars and lorries and have a big Jeep!</p>
  <h2>Where I live</h2>
  <p>I live in a small hut on a mountain!</p>
 </body>
</html>

Because HTML conveys meaning, rather than presentation, the same page can also be used by a small browser on a mobile phone, without any change to the page. Instead of headings being in large letters as on the desktop, for example, the browser on the mobile phone might use the same size text for the whole page, but with the headings in bold.

But it goes further than just differences in screen size: the same page could equally be used by a blind user using a browser based around speech synthesis, which instead of displaying the page on a screen, reads the page to the user, e.g. using headphones. Instead of large text for the headings, the speech browser might use a different volume or a slower voice.

That's not all, either. Since the browsers know which parts of the page are the headings, they can create a document outline that the user can use to quickly navigate around the document, using keys for "jump to next heading" or "jump to previous heading". Such features are especially common with speech browsers, where users would otherwise find quickly navigating a page quite difficult.

Even beyond browsers, software can make use of this information. Search engines can use the headings to more effectively index a page, or to provide quick links to subsections of the page from their results. Tools can use the headings to create a table of contents (that is in fact how this very specification's table of contents is generated).

This example has focused on headings, but the same principle applies to all of the semantics in HTML.

Authors must not use elements, attributes, or attribute values for purposes other than their appropriate intended semantic purpose, as doing so prevents software from correctly processing the page.

For example, the following snippet, intended to represent the heading of a corporate site, is non-conforming because the second line is not intended to be a heading of a subsection, but merely a subheading or subtitle (a subordinate heading for the same section).

<body>
 <h1>ACME Corporation</h1>
 <h2>The leaders in arbitrary fast delivery since 1920</h2>
 ...

The hgroup element can be used for these kinds of situations:

<body>
 <hgroup>
  <h1>ACME Corporation</h1>
  <p>The leaders in arbitrary fast delivery since 1920</p>
 </hgroup>
 ...

The document in this next example is similarly non-conforming, despite being syntactically correct, because the data placed in the cells is clearly not tabular data, and the cite element mis-used:

<!DOCTYPE HTML>
<html lang="en-GB">
 <head> <title> Demonstration </title> </head>
 <body>
  <table>
   <tr> <td> My favourite animal is the cat. </td> </tr>
   <tr>
    <td><a href="https://example.org/~ernest/"><cite>Ernest</cite></a>,
     in an essay from 1992
    </td>
   </tr>
  </table>
 </body>
</html>

This would make software that relies on these semantics fail: for example, a speech browser that allowed a blind user to navigate tables in the document would report the quote above as a table, confusing the user; similarly, a tool that extracted titles of works from pages would extract "Ernest" as the title of a work, even though it's actually a person's name, not a title.

A corrected version of this document might be:

<!DOCTYPE HTML>
<html lang="en-GB">
 <head> <title> Demonstration </title> </head>
 <body>