Living Standard — Last Updated 6 June 2023
Support in all current engines.
This section only describes the rules for XML resources. Rules for
text/html resources are discussed in the section above entitled "The HTML
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 syntax for XML is defined in XML and Namespaces in XML. [XML] [XMLNS]
This specification does not define any syntax-level requirements beyond those defined for XML proper.
XML documents may contain a
DOCTYPE if desired, but this is not required
to conform to this specification. This specification does not define a public or system
identifier, nor provide a formal DTD.
According to XML, XML processors are not guaranteed to process
the external DTD subset referenced in the DOCTYPE. This means, for example, that using entity references for characters in XML documents
is unsafe if they are defined in an external file (except for
This section describes the relationship between XML and the DOM, with a particular emphasis on how this interacts with HTML.
An XML parser, for the purposes of this specification, is a construct that
follows the rules given in XML to map a string of bytes or characters into a
At the time of writing, no such rules actually exist.
An XML parser is either associated with a
Document object when it is
created, or creates one implicitly.
Document must then be populated with DOM nodes that represent the tree
structure of the input passed to the parser, as defined by XML, Namespaces
in XML, and DOM. When creating DOM nodes representing elements,
the create an element for a token algorithm
or some equivalent that operates on appropriate XML data structures must be used, to ensure the
proper element interfaces are created and that custom elements are set up correctly.
DOM mutation events must not fire for the operations that the XML parser performs
Document's tree, but the user agent must act as if elements and attributes
were individually appended and set respectively so as to trigger rules in this specification
regarding what happens when an element is inserted into a document or has its attributes set, and
DOM's requirements regarding mutation observers mean that
mutation observers are fired (unlike mutation events). [XML] [XMLNS]
Between the time an element's start tag is parsed and the time either the element's end tag is parsed or the parser detects a well-formedness error, the user agent must act as if the element was in a stack of open elements.
This is used by various elements to only start certain processes once they are popped off of the stack of open elements.
This specification provides the following additional information that user agents should use when retrieving an external entity: the public identifiers given in the following list all correspond to the URL given by this link. (This URL is a DTD containing the entity declarations for the names listed in the named character references section.) [XML]
-//W3C//DTD XHTML 1.0 Transitional//EN
-//W3C//DTD XHTML 1.1//EN
-//W3C//DTD XHTML 1.0 Strict//EN
-//W3C//DTD XHTML 1.0 Frameset//EN
-//W3C//DTD XHTML Basic 1.0//EN
-//W3C//DTD XHTML 1.1 plus MathML 2.0//EN
-//W3C//DTD XHTML 1.1 plus MathML 2.0 plus SVG 1.1//EN
-//W3C//DTD MathML 2.0//EN
-//WAPFORUM//DTD XHTML Mobile 1.0//EN
Furthermore, user agents should attempt to retrieve the above external entity's content when one of the above public identifiers is used, and should not attempt to retrieve any other external entity's content.
This is not strictly a violation of XML, but it does contradict the spirit of XML's requirements. This is motivated by a desire for user agents to all handle entities in an interoperable fashion without requiring any network access for handling external subsets. [XML]
XML parsers can be invoked with XML scripting support enabled or XML scripting support disabled. Except where otherwise specified, XML parsers are invoked with XML scripting support enabled.
When an XML parser with XML scripting support
enabled creates a
script element, it must have its parser
document set and its force async set to false. If
the parser was created as part of the XML fragment parsing algorithm, then the
element's already started must be set to true. When the element's end tag is
subsequently parsed, the user agent must perform a microtask checkpoint, and then
script element. If this
causes there to be a pending parsing-blocking script, then the user agent must run
the following steps:
Block this instance of the XML parser, such that the event loop will not run tasks that invoke it.
Spin the event loop until the parser's
Document has no
style sheet that is blocking scripts and the pending parsing-blocking
script's ready to be parser-executed is true.
Unblock this instance of the XML parser, such that tasks that invoke it can again be run.
Execute the script element given by the pending parsing-blocking script.
Set the pending parsing-blocking script to null.
document.write() API is not
available for XML documents, much of the complexity in the HTML parser
is not needed in the XML parser.
When the XML parser has XML scripting support disabled, none of this happens.
When an XML parser would append a node to a
template element, it must instead append it to the
template contents (a
This is a willful violation of XML; unfortunately,
XML is not formally extensible in the manner that is needed for
When an XML parser creates a
Node object, its node document
must be set to the node document of
the node into which the newly created node is to be inserted.
Certain algorithms in this specification spoon-feed the parser characters one string at a time. In such cases, the XML parser must act as it would have if faced with a single string consisting of the concatenation of all those characters.
When an XML parser reaches the end of its input, it must stop parsing, following the same rules as the HTML parser. An XML parser can also be aborted, which must again be done in the same way as for an HTML parser.
For the purposes of conformance checkers, if a resource is determined to be in the XML syntax, then it is an XML document.
The XML fragment serialization
algorithm for a
Element node either returns a fragment
of XML that represents that node or throws an exception.
Documents, the algorithm must return a string in the form of a document entity, if none of the error cases
Elements, the algorithm must return a string in the form of an internal general parsed entity, if none of the
error cases below apply.
In both cases, the string returned must be XML namespace-well-formed and must be an isomorphic
serialization of all of that node's relevant child nodes, in tree order.
User agents may adjust prefixes and namespace declarations in the serialization (and indeed might
be forced to do so in some cases to obtain namespace-well-formed XML). User agents may use a
combination of regular text and character references to represent
Text nodes in the
A node's relevant child nodes are those that apply given the following rules:
templateelement's template contents, if any.
Elements, if any of the elements in the serialization are in no namespace, the
default namespace in scope for those elements must be explicitly declared as the empty string. (This doesn't apply in the
Document case.) [XML]
For the purposes of this section, an internal general parsed entity is considered XML namespace-well-formed if a document consisting of an element with no namespace declarations whose contents are the internal general parsed entity would itself be XML namespace-well-formed.
If any of the following error cases are found in the DOM subtree being serialized, then the
algorithm must throw an "
instead of returning a string:
Documentnode with no child element nodes.
DocumentTypenode that has an external subset public identifier that contains characters that are not matched by the XML
DocumentTypenode that has an external subset system identifier that contains both a U+0022 QUOTATION MARK (") and a U+0027 APOSTROPHE (') or that contains characters that are not matched by the XML
Attrnode with no namespace whose local name is the lowercase string "
Elementnode with two or more attributes with the same local name and namespace.
ProcessingInstructionnode whose data contains characters that are not matched by the XML
Commentnode whose data contains two adjacent U+002D HYPHEN-MINUS characters (-) or ends with such a character.
ProcessingInstructionnode whose target name is an ASCII case-insensitive match for the string "
ProcessingInstructionnode whose target name contains a U+003A COLON (:).
ProcessingInstructionnode whose data contains the string "
These are the only ways to make a DOM unserialisable. The DOM enforces all the
other XML constraints; for example, trying to append two elements to a
will throw a "
The XML fragment parsing algorithm either returns a
Document or throws
DOMException. Given a string
input and a context element context, the
algorithm is as follows:
Create a new XML parser.
Feed the parser just created the string corresponding to the start tag of the context element, declaring all the namespace prefixes that are in scope on that element in the DOM, as well as declaring the default namespace (if any) that is in scope on that element in the DOM.
A namespace prefix is in scope if the DOM
on the element would return a non-null value for that prefix.
The default namespace is the namespace for which the DOM
isDefaultNamespace() method on the element would return true.
DOCTYPE is passed to the parser, and therefore no external subset is
referenced, and therefore no entities will be recognized.
Feed the parser just created the string input.
Feed the parser just created the string corresponding to the end tag of the context element.
If there is an XML well-formedness or XML namespace well-formedness error, then throw a
If the document element of the resulting
Document has any sibling
nodes, then throw a "
Return the child nodes of the document element of the resulting
Document, in tree order.