This specification depends on the WHATWG Infra standard. [INFRA]
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
objects and their descendant DOM trees, and to serialized byte streams using the HTML syntax or the XML syntax, depending
In the context of the DOM structures, the terms HTML
document and XML document are used as defined in the DOM
specification, and refer specifically to two different modes that
can find themselves in. [DOM] (Such uses are always hyperlinked to their
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
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.
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 specification, 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.
To ease migration from HTML to XML, UAs 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
button, meaning they have the local name "
(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]
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 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,
or string, means that the length of the text is zero (i.e., not even containing controls or U+0020 SPACE).
A node A is inserted into a node B when the insertion steps are invoked with A as the argument and A's new parent is B. Similarly, a node A is removed from a node B when the removing steps are invoked with A as the removedNode argument and B as the oldParent argument.
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 has a browsing context. 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 no longer has a browsing context.
The construction "a
Foo object", where
actually an interface, is sometimes used instead of the more accurate "an object implementing the
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 operate on the actual underlying data, not a snapshot of the data.
The term plugin refers to a user-agent 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 browsing contexts of the
Document nor introduce any
Node objects to the
Typically such content handlers are provided by third parties, though a user agent can also designate built-in content handlers as plugins.
One example of a plugin would be a PDF viewer that is instantiated in a browsing context 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]
A plugin can be secured if it honors the semantics of
For example, a secured plugin would prevent its contents from creating pop-up
windows when the plugin is instantiated inside a sandboxed
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 the WHATWG Encoding standard. 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]
A UTF-16 encoding is UTF-16BE or UTF-16LE. [ENCODING]
An ASCII-compatible encoding is any encoding that is not a UTF-16 encoding. [ENCODING]
Since support for encodings that are not defined in the WHATWG Encoding standard is prohibited, UTF-16 encodings are the only encodings that this specification needs to treat as not being ASCII-compatible encodings.
This specification relies on several other underlying specifications.
The following terms are defined in the WHATWG Infra standard: [INFRA]
This specification introduces terminology based on the terms defined in those specifications, as described earlier.
The following terms are used as defined in the WHATWG Encoding standard: [ENCODING]
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.
This specification also non-normatively mentions the
interface and its
transformToDocument() methods. [XSLTP]
The following terms are defined in the WHATWG URL standard: [URL]
A number of schemes and protocols are referenced by this specification also:
The following terms are defined in the HTTP specifications: [HTTP]
The following terms are defined in the Cookie specification: [COOKIES]
The following term is defined in the Web Linking specification: [WEBLINK]
The following terms are defined in the WHATWG MIME Sniffing standard: [MIMESNIFF]
The following terms are defined in the WHATWG Fetch standard: [FETCH]
The following terms are defined in Referrer Policy: [REFERRERPOLICY]
Referrer-Policy` HTTP header
Referrer-Policy` header algorithm
no-referrer-when-downgrade", and "
unsafe-url" referrer policies
The following terms are defined in Mixed Content: [MIX]
The IDL fragments in this specification must be interpreted as required for conforming IDL fragments, as described in the Web IDL specification. [WEBIDL]
The following terms are defined in the Web IDL specification:
The Web IDL specification also defines the following types that are used in Web IDL fragments in this specification:
When this specification requires a user agent to create a
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
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
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
Date object that represents a time value NaN (indicating that the object does
not represent a specific instant of time).
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 the WHATWG DOM standard: [DOM]
Node, and the concept of cloning steps used by that algorithm
MutationObserverinterface and mutation observers in general
The following features are defined in the UI Events specification: [UIEVENTS]
The following features are defined in the Touch Events specification: [TOUCH]
The following features are defined in the Pointer Events specification: [POINTEREVENTS]
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 the DOM Parsing and Serialization specification: [DOMPARSING]
User agents are encouraged to implement the features described in the execCommand specification. [EXECCOMMAND]
The following parts of the WHATWG Fullscreen API standard are referenced from this
specification, in part to define the rendering of
dialog elements, and also to
define how the Fullscreen API interacts with HTML: [FULLSCREEN]
This specification uses the following features defined in the File API specification: [FILEAPI]
The following terms are defined in the Media Source Extensions specification: [MEDIASOURCE]
The following terms are defined in the Media Capture and Streams specification: [MEDIASTREAM]
This specification references the XMLHttpRequest specification to describe how the two
specifications interact and to use its
ProgressEvent features. The following
features and terms are defined in the XMLHttpRequest specification: [XHR]
The following features are defined in the Battery Status API specification: [BATTERY]
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 the CSS Syntax specification 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.
To parse a CSS <color> value, given a string input with an optional element element, run these steps:
If color is failure, then return failure.
If color is
The following terms and features are defined in the CSS specification: [CSS]
The CSS specification also defines the following border properties: [CSS]
The terms intrinsic width and intrinsic height refer to the width dimension and the height dimension, respectively, of intrinsic dimensions.
The following terms and features are defined in the CSS Logical Properties specification: [CSSLOGICAL]
The following terms and features are defined in the CSS Color specification: [CSSCOLOR]
The following features are defined in the CSS Backgrounds and Borders specification: [CSSBG]
The following features are defined in the CSS Fonts specification: [CSSFONTS]
The following features are defined in the CSS Positioned Layout specification: [CSSPOSITION]
The following features are defined in the CSS Table specification: [CSSTABLE]
The following features are defined in the CSS Text specification: [CSSTEXT]
The following features are defined in the CSS Writing Modes specification: [CSSWM]
The following features are defined in the CSS Basic User Interface specification: [CSSUI]
The following features and terms are defined in the CSS Syntax specifications: [CSSSYNTAX]
The following terms are defined in the Selectors specification: [SELECTORS]
The following features are defined in the CSS Values and Units specification: [CSSVALUES]
The following terms are defined in the CSS Cascading and Inheritance specification: [CSSCASCADE]
CanvasRenderingContext2D object's use of fonts depends on the features
described in the CSS Fonts and Font Loading specifications, including
FontFace objects and the font source concept.
The following interfaces and terms are defined in the Geometry Interfaces Module specification: [GEOMETRY]
The following term is defined in the Intersection Observer specification: [INTERSECTIONOBSERVER]
The following interface is defined in the WebGL specification: [WEBGL]
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 the WebVTT specification:
The following terms are defined in the WHATWG Fetch standard: [FETCH]
The following terms are defined in the WebSocket protocol specification: [WSP]
role attribute is defined in the ARIA
specification, as are the following roles: [ARIA]
In addition, the following
attributes are defined in the ARIA specification: [ARIA]
Finally, the following terms are defined in the ARIA specification: [ARIA]
The following terms are defined in Content Security Policy: [CSP]
The following terms are defined in Service Workers: [SW]
The following terms are defined in Secure Contexts: [SECURE-CONTEXTS]
The following feature is defined in the Payment Request API specification: [PAYMENTREQUEST]
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 the MathML specification:
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.
Also, the SVG specifications do not reflect implementation reality. Implementations implement subsets of SVG 1.1 and SVG Tiny 1.2. Although it is hoped that the in-progress SVG 2 specification is a more realistic target for implementations, until that specification is ready, user agents that implement SVG must do so with the following willful violations and additions. [SVG] [SVGTINY12] [SVG2]
User agents that implement SVG must not implement the following features from SVG 1.1:
cursorelement (use CSS's
contentStyleTypeattributes (use the
typeattribute on the SVG
User agents that implement SVG must implement the following features from SVG Tiny 1.2:
non-scaling-strokevalue for the
classattribute is allowed on all SVG elements
tabindexattribute is allowed on visible SVG elements
The following features are defined in the SVG specifications:
The following feature is defined in the Filter Effects specification: [FILTERS]
The following feature is defined in the Worklets specification: [WORKLETS]
This specification might have certain additional requirements on character encodings, image formats, audio formats, and video formats in the respective sections.
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.
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.
Comparing two strings in a case-sensitive manner means comparing them exactly, code point for code point.
Except where otherwise stated, string comparisons must be performed in a case-sensitive manner.
A string pattern is a prefix match for a string s when pattern is not longer than s and truncating s to pattern's length leaves the two strings as matches of each other.