Generally, when the specification states that a feature applies to the HTML syntax or the XHTML 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 XHTML)".
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
Document objects and their descendant DOM
trees, and to serialized byte streams using the HTML syntax or
XHTML syntax, depending on
In the context of the DOM structures, the terms HTML document and XML document are used as defined in the
DOM Core specification, and refer specifically to two different
Document objects can find themselves in.
(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.
The term XHTML document is used
to refer to both
Documents in the XML document mode that contains element
nodes in the HTML namespace, and byte streams labeled
with an XML MIME type that contain elements from the
HTML namespace, depending on context.
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 term "transparent black" refers to the color with red, green, blue, and alpha channels all set to zero.
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 and URI specifications, refer to as a representation is referred to in this specification as a resource. [HTTP] [RFC3986]
The term MIME type is used to refer to what is sometimes called an Internet media type in protocol literature. The term media type in this specification is used to refer to the type of media intended for presentation, as used by the CSS specifications. [RFC2046] [MQ]
A string is a valid MIME type if
it matches the
media-type rule defined in
section 3.7 "Media Types" of RFC 2616. In particular, a valid MIME type may include MIME type
A string is a valid
MIME type with no parameters if it matches the
media-type rule defined in section 3.7 "Media Types" of
RFC 2616, but does not contain any ";" (U+003B) characters. In
other words, if it consists only of a type and subtype, with no
MIME Type parameters. [HTTP]
The term HTML MIME type is used
to refer to the MIME type
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.
data: URL refers to
URLs that use the
data: scheme. [RFC2397]
To ease migration from HTML to XHTML, 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", when used in this
specification, refers to any element in that namespace, and thus
refers to both HTML and XHTML elements.
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
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, they contain no ":"
(U+003A) characters, and their first three characters are not an
ASCII case-insensitive match for
the string "
The term XML MIME type is used to
refer to the MIME types
application/xml, and any
MIME type whose subtype ends with the four
The root element of
Document object is that
Document's first element child, if any. If
it does not have one then the
Document has no root element.
The term root element, when not
referring to a
Document object's root element, means the
furthest ancestor element node of whatever node is being discussed,
or the node itself if it has no ancestors. When the node is a part
of the document, then the node's root element is indeed the document's root
element; however, if the node is not currently part of the document
tree, the root element will be an orphaned node.
When an element's root element is the root element of a
Document object, it is said to be in a
Document. An element is
said to have been inserted into a
document when its root element changes and is now the document's
root element. Analogously, an element is said
to have been removed from a document
when its root element changes from being the document's
root element to being another element.
A node's home subtree is the
subtree rooted at that node's root element. When a node is in a
Document, its home subtree is that
Document of a
(such as an element) is the
Document that the
ownerDocument IDL attribute returns. When a
is in a
Document then that
Document is always the
Document, and the
ownerDocument IDL attribute thus always returns that
Document of a content attribute is the
Document of the attribute's element.
The term tree order means a
pre-order, depth-first traversal of DOM nodes involved (through the
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 of an attribute
node, or string, means that the length of the text is zero (i.e.
not even containing spaces or control characters).
The construction "a
Foo object", where
Foo is actually an interface, is sometimes used
instead of the more accurate "an object implementing the interface
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.
In the contexts of events, the terms fire
and dispatch are used as defined in the
DOM Core specification: firing an event means to create and
dispatch it, and dispatching an event means to follow the steps
that propagate the event through the tree. The term trusted
event is used to refer to events whose
isTrusted attribute is initialized to true. [DOMCORE]
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
browsing contexts of the
Document nor introduce any
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
The preferred MIME name of a character encoding is the name or alias labeled as "preferred MIME name" in the IANA Character Sets registry, if there is one, or the encoding's name, if none of the aliases are so labeled. [IANACHARSET]
An ASCII-compatible character encoding is a single-byte or variable-length encoding in which the bytes 0x09, 0x0A, 0x0C, 0x0D, 0x20 - 0x22, 0x26, 0x27, 0x2C - 0x3F, 0x41 - 0x5A, and 0x61 - 0x7A , ignoring bytes that are the second and later bytes of multibyte sequences, all correspond to single-byte sequences that map to the same Unicode characters as those bytes in ANSI_X3.4-1968 (US-ASCII). [RFC1345]
This includes such encodings as Shift_JIS, HZ-GB-2312, and variants of ISO-2022, even though it is possible in these encodings for bytes like 0x70 to be part of longer sequences that are unrelated to their interpretation as ASCII. It excludes such encodings as UTF-7, UTF-16, GSM03.38, and EBCDIC variants.
The term a UTF-16 encoding refers to any variant of UTF-16: self-describing UTF-16 with a BOM, ambiguous UTF-16 without a BOM, raw UTF-16LE, and raw UTF-16BE. [RFC2781]
The term code unit is used as defined
in the Web IDL specification: a 16 bit unsigned integer, the
smallest atomic component of a
DOMString. (This is a
narrower definition than the one used in Unicode.) [WEBIDL]
The term Unicode code point means a Unicode scalar value where possible, and an isolated surrogate code point when not. When a conformance requirement is defined in terms of characters or Unicode code points, a pair of code units consisting of a high surrogate followed by a low surrogate must be treated as the single code point represented by the surrogate pair, but isolated surrogates must each be treated as the single code point with the value of the surrogate. [UNICODE]
In this specification, the term character, when not qualified as Unicode character, is synonymous with the term Unicode code point.
The term Unicode character is used to mean a Unicode scalar value (i.e. any Unicode code point that is not a surrogate code point). [UNICODE]
The code-unit length of a string is the number of code units in that string.
This complexity results from the historical decision to define the DOM API in terms of 16 bit (UTF-16) code units, rather than in terms of Unicode characters.
All diagrams, examples, and notes in this specification are non-normative, as are all sections explicitly marked non-normative. Everything else in this specification is normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document are to be interpreted as described in RFC2119. The key word "OPTIONALLY" in the normative parts of this document is to be interpreted with the same normative meaning as "MAY" and "OPTIONAL". For readability, these words do not appear in all uppercase letters in this specification. [RFC2119]
HTML has a wide number of extensibility mechanisms that can be used for adding semantics in a safe manner:
classattribute to extend elements, effectively creating their own elements, while using the most applicable existing "real" HTML element, so that browsers and other tools that don't know of the extension can still support it somewhat well. This is the tack used by microformats, for example.
data-*=""attributes. These are guaranteed to never be touched by browsers, and allow scripts to include data on HTML elements that scripts can then look for and process.
<meta name="" content="">mechanism to include page-wide metadata by registering extensions to the predefined set of metadata names.
rel=""mechanism to annotate links with specific meanings by registering extensions to the predefined set of link types. This is also used by microformats. Additionally, absolute URLs that do not contain any non-ASCII characters, nor characters in the range U+0041 (LATIN CAPITAL LETTER A) through U+005A (LATIN CAPITAL LETTER Z) (inclusive), may be used as link types.
<script type="">mechanism with a custom type, for further handling by inline or server-side scripts.
embedelement. This is how Flash works.
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.
The conformance terminology for documents depends on the nature of the changes introduced by such applicable specifications, and on the content and intended interpretation of the document. Applicable specifications MAY define new document content (e.g. a foobar element), MAY prohibit certain otherwise conforming content (e.g. prohibit use of <table>s), or MAY change the semantics, DOM mappings, or other processing rules for content defined in this specification. Whether a document is or is not a conforming HTML5 document does not depend on the use of applicable specifications: if the syntax and semantics of a given conforming HTML5 document is unchanged by the use of applicable specification(s), then that document remains a conforming HTML5 document. If the semantics or processing of a given (otherwise conforming) document is changed by use of applicable specification(s), then it is not a conforming HTML5 document. For such cases, the applicable specifications SHOULD define conformance terminology.
As a suggested but not required convention, such specifications might define conformance terminology such as: "Conforming HTML5+XXX document", where XXX is a short name for the applicable specification. (Example: "Conforming HTML5+AutomotiveExtensions document").
a consequence of the rule given above is that certain syntactically correct HTML5 documents may not be conforming HTML5 documents in the presence of applicable specifications. (Example: the applicable specification defines <table> to be a piece of furniture — a document written to that specification and containing a <table> element is NOT a conforming HTML5 document, even if the element happens to be syntactically correct HTML5.)
Comparing two strings in a case-sensitive manner means comparing them exactly, code point for code point.
Comparing two strings in an ASCII case-insensitive manner means comparing them exactly, code point for code point, except that the characters in the range U+0041 to U+005A (i.e. LATIN CAPITAL LETTER A to LATIN CAPITAL LETTER Z) and the corresponding characters in the range U+0061 to U+007A (i.e. LATIN SMALL LETTER A to LATIN SMALL LETTER Z) are considered to also match.
Comparing two strings in a compatibility caseless manner means using the Unicode compatibility caseless match operation to compare the two strings. [UNICODE]
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.