Support in all current engines.
本章只描述 XML 资源的语法规则。
text/html 资源的规则在上一章 "HTML
语法" 中讨论。
HTML 的 XML 语法以前称为 "XHTML"，但本规范不适用这一术语 （一个原因是这样的术语不能用于 MathML 和 SVG 的 HTML 语法）。
XML 的语法定义在 XML 和 Namespaces in XML 规范中。[XML] [XMLNS]
本规范没有在 XML 规范的基础上定义任何语法级别的要求。
XML 文档可能会包含一个 DOCTYPE，但符合本规范不要求包含。
本规范没有定义共有或系统标识符，也没有提供正式的 DTD。
根据 XML 规范，XML 处理器不保证处理 DOCTYPE 中引用的外部 DTD 子集。
这意味着，例如为 XML 文档中的字符使用
实体引用
时，如果它们定义在外部文件中（
除了 <，>，
&， " 以及
' 之外
）就是不安全的。
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 the XML specification to map a string of bytes or characters into
a Document object.
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.
This Document must then be populated with DOM nodes that represent the tree
structure of the input passed to the parser, as defined by the XML specification, the Namespaces
in XML specification, and the WHATWG DOM standard. When creating DOM nodes representing elements,
the create an element for a token algorithm
or some equivalent that operates on appropriate XML datastructures 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
on the 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
the WHATWG DOM standard's requirements regarding mutation observers mean that
mutation observers are fired (unlike mutation events). [XML] [XMLNS]
[DOM] [UIEVENTS]
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, e.g. by the object element to avoid instantiating plugins
before the param element children have been parsed.
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//ENFurthermore, 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 the XML specification, but it does contradict the spirit of the XML specification'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 be marked as being "parser-inserted" and its "non-blocking" flag
must be unset. If the parser was originally created for the XML fragment parsing
algorithm, then the element must be marked as "already started" also. When the
element's end tag is subsequently parsed, the user agent must perform a microtask checkpoint, and
then prepare the 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" flag is set.
Unblock this instance of the XML parser, such that tasks that invoke it can again be run.
There is no longer a pending parsing-blocking script.
Since the 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 element's
template contents (a DocumentFragment node).
This is a willful violation of the XML specification; unfortunately,
XML is not formally extensible in the manner that is needed for template processing.
[XML]
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 XML 语法, then it is an XML document.
The XML fragment serialization
algorithm for a Document or Element node either returns a fragment
of XML that represents that node or throws an exception.
For Documents, the algorithm must return a string in the form of a document entity, if none of the error cases
below apply.
For 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
DOM.
A node's relevant child nodes are those that apply given the following rules:
template elementstemplate
element's template contents, if any.For 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]
[XMLNS]
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 "InvalidStateError" DOMException
instead of returning a string:
Document node with no child element nodes.DocumentType node that has an external subset public identifier that contains
characters that are not matched by the XML PubidChar production. [XML]DocumentType node 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 Char production. [XML]Name production. [XML]Attr node with no namespace whose local name is the lowercase string "xmlns". [XMLNS]Element node with two or more attributes with the same local name and
namespace.Attr node, Text node, Comment node, or
ProcessingInstruction node whose data contains characters that are not matched by
the XML Char production. [XML]Comment node whose data contains two adjacent U+002D HYPHEN-MINUS characters
(-) or ends with such a character.ProcessingInstruction node whose target name is an ASCII
case-insensitive match for the string "xml".ProcessingInstruction node whose target name contains a U+003A COLON (:).ProcessingInstruction node 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 Document node
will throw a "HierarchyRequestError" DOMException.
The XML fragment parsing algorithm either returns a Document or throws
a "SyntaxError" 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 lookupNamespaceURI() method
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.
No
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
"SyntaxError" DOMException.
If the document element of the resulting Document has any sibling
nodes, then throw a "SyntaxError" DOMException.
Return the child nodes of the document element of the resulting
Document, in tree order.