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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<head>
<title>Selectors Level 4</title>
<link href="../default.css" type="text/css" rel="stylesheet">
<style type="text/css">
.tprofile td, th { vertical-align: baseline; padding: 0 0.5em; }
.tprofile th { text-align: right; }
</style>
<link href="http://www.w3.org/StyleSheets/TR/W3C-WD.css" type="text/css" rel="stylesheet">
</head>
<body>
<div class="head">
<!--logo-->
<h1 id=title>Selectors Level 4</h1>
<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
<dt>This version:
<!--
<dd><a href="[VERSION]">
http://www.w3.org/TR/[YEAR]/WD-[SHORTNAME]-[CDATE]/</a>
<dt>Editor's draft:
-->
<dd><a href="http://dev.w3.org/csswg/selectors4/">
http://dev.w3.org/csswg/selectors4/</a>
<dt>Latest version of Selectors Level 4:
<dd><a href="http://www.w3.org/TR/selectors4/">
http://www.w3.org/TR/selectors4/</a>
<dt>Latest Selectors specification:
<dd><a href="http://www.w3.org/TR/selectors/">
http://www.w3.org/TR/selectors/</a>
<dt>Previous version:
<dd><a href="http://www.w3.org/TR/2011/WD-selectors4-20110929/">
http://www.w3.org/TR/2011/WD-selectors4-20110929/</a>
<dt>Editors:
<dd class="vcard"><a class="url fn" href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a> (Mozilla)
<dt>Previous Editors:
<dd class="vcard"><a lang="tr" class="url fn" href="http://www.tantek.com/">Tantek Çelik</a> (Microsoft)
<dd class="vcard"><span class="fn">Daniel Glazman</span> (Disruptive Innovations SARL)</dd>
<dd class="vcard"><span class="fn">Ian Hickson</span> (Opera Softare ASA)
<dd class="vcard"><span class="fn">Peter Linss</span> (Netscape/AOL)
<dd class="vcard"><span class="fn">John Williams</span> (Quark, Inc.)
</dl>
<!--copyright-->
<hr title="Separator for header">
</div>
<h2 class="no-num no-toc" id=abstract>
Abstract</h2>
<p><em>Selectors</em> are patterns that match against elements in a
tree, and as such form one of several technologies that can be used
to select nodes in an XML document. Selectors have been optimized for
use with HTML and XML, and are designed to be usable in
performance-critical code. They are a core component of
<abbr title="Cascading Style Sheets">CSS</abbr> (Cascading
Style Sheets), which uses Selectors to bind style properties to
elements in the document.</p>
<p>Selectors Level 4 describes the selectors that already exist in
[[!SELECT]], and further introduces new selectors for CSS and
other languages that may need them.</p>
<h2 class="no-num no-toc" id=status>Status of this Document</h2>
<!--status-->
<p><strong>This module is an early-stage Working Draft. If you are
looking for a stable Selectors specification, use
<a href="http://www.w3.org/TR/css3-selectors/">Selectors 3</a>.</strong>
Read the <a href="http://www.w3.org/TR/CSS/">CSS Snapshot</a> for
an overview of the CSS development process.
See the <a href="#overview">Selectors Overview</a> for a summary of
additions to level 3.
<p>This is the first public Working Draft of this new specification.
<h2 class="no-num no-toc" id="contents">Table of Contents</h2>
<!--toc-->
<h2 id=context>
Introduction</h2>
<p><em>This section is not normative.</em>
<p>Selectors define the following function for elements in a tree structure:</p>
<pre>expression ∗ element → boolean</pre>
<p>That is, given an element and a selector, this specification
defines whether that element matches the selector.</p>
<p>These expressions can also be used, for instance, to select a set
of elements, or a single element from a set of elements, by
evaluating the expression across all the elements in a
subtree. <abbr title="Simple Tree Transformation
Sheets">STTS</abbr> (Simple Tree Transformation Sheets), a
language for transforming XML trees, uses this mechanism. [[STTS3]]</p>
<p>Selectors Levels 1, 2, and 3 are defined as the subsets of selector
functionality defined in the <a href="http://www.w3.org/TR/REC-CSS1">CSS1</a>,
<a href="http://www.w3.org/TR/CSS21/">CSS2.1</a>, and
<a href="http://www.w3.org/TR/css3-selectors/">Selectors Level 3</a>
specifications, respectively. This module defines Selectors Level 4.
<h3 id="placement">Module Interactions</h3>
<p>This module replaces the definitions for, and extends the set of
selectors defined for CSS
in [[SELECT]] and [[CSS21]].
<p>Pseudo-element selectors, which define abstract elements in a
rendering tree, are not part of this specification: their generic syntax
is described here, but, due to their close integration with the rendering
model and irrelevance to other uses such as DOM queries, they will be
defined in other modules.
<h2 id=overview>
Selectors Overview</h2>
<p><em>This section is non-normative, as it merely summarizes the
following sections.</em></p>
<p>A Selector represents a structure. This structure can be used as a
condition (e.g. in a CSS rule) that determines which elements a
selector matches in the document tree, or as a flat description of the
HTML or XML fragment corresponding to that structure.</p>
<p>Selectors may range from simple element names to rich contextual
representations.</p>
<p>The following table sum
77C
marizes the Selector syntax:</p>
<table class="data">
<col class="pattern">
<col class="meaning">
<col class="section">
<col class="level">
<thead>
10444
<tr>
<th>Pattern
<th>Meaning
<th>Section
<th>Level
<tbody>
<tr>
<td><code>*</code>
<td>any element
<td><a href="#universal-selector">Universal selector</a>
<td>2
<tr>
<td><code>E</code>
<td>an element of type E
<td><a href="#type-selectors">Type selector</a>
<td>1
<tbody>
<tr>
<td><code>E:not(<var>s1</var>, <var>s2</var>)</code>
<td>an E element that does not match either compound selector <var>s1</var>
or compound selector <var>s2</var>
<td><a href="#negation">Negation pseudo-class</a>
<td>3/4
<tr>
<td><code>E:matches(<var>s1</var>, <var>s2</var>)</code>
<td>an E element that matches compound selector <var>s1</var>
and/or compound selector <var>s2</var>
<td><a href="#matches">Matches-any pseudo-class</a>
<td>4
<tbody>
<tr>
<td><code>E.warning</code>
<td>an E element belonging to the class <code>warning</code>
(the document language specifies how class is determined).
<td><a href="#class-html">Class selectors</a>
<td>1
<tr>
<td><code>E#myid</code>
<td>an E element with ID equal to <code>myid</code>.
<td><a href="#id-selectors">ID selectors</a>
<td>1
<tr>
<td><code>E[foo]</code>
<td>an E element with a <code>foo</code> attribute
<td><a href="#attribute-selectors">Attribute selectors</a>
<td>2
<tr>
<td><code>E[foo="bar"]</code>
<td>an E element whose <code>foo</code> attribute value is
exactly equal to <code>bar</code>
<td><a href="#attribute-selectors">Attribute selectors</a>
<td>2
<tr>
<td><code>E[foo="bar" i]</code>
<td>an E element whose <code>foo</code> attribute value is
exactly equal to any (ASCII-range) case-permutation of <code>bar</code>
<td><a href="#attribute-case">Attribute selectors: Case-sensitivity</a>
<td>4
<tr>
<td><code>E[foo~="bar"]</code>
<td>an E element whose <code>foo</code> attribute value is
a list of whitespace-separated values, one of which is
exactly equal to <code>bar</code>
<td><a href="#attribute-selectors">Attribute selectors</a>
<td>2
<tr>
<td><code>E[foo^="bar"]</code>
<td>an E element whose <code>foo</code> attribute value
begins exactly with the string "bar"
<td><a href="#attribute-selectors">Attribute selectors</a>
<td>3
<tr>
<td><code>E[foo$="bar"]</code>
<td>an E element whose <code>foo</code> attribute value
ends exactly with the string <code>bar</code>
<td><a href="#attribute-selectors">Attribute selectors</a>
<td>3
<tr>
<td><code>E[foo*="bar"]</code>
<td>an E element whose <code>foo</code> attribute value
contains the substring <code>bar</code>
<td><a href="#attribute-selectors">Attribute selectors</a>
<td>3
<tr>
<td><code>E[foo|="en"]</code>
<td>an E element whose <code>foo</code> attribute value is
a hyphen-separated list of values beginning with <code>en</code>
<td><a href="#attribute-selectors">Attribute selectors</a>
<td>2
<tbody>
<tr>
<td><code>E:dir(ltr)</code>
<td>an element of type E in with left-to-right directionality
(the document language specifies how directionality is determined)
<td><a href="#dir-pseudo">The :dir() pseudo-class</a>
<td>4
<tr>
<td><code>E:lang(fr, en)</code>
<td>an element of type E in either language "fr" or language "en"
(the document language specifies how language is determined)
<td><a href="#lang-pseudo">The :lang() pseudo-class</a>
<td>2/4
<tbody>
<tr>
<td><code>E:any-link</code>
<td>an E element being the source anchor of a hyperlink
<td><a href="#any-link-pseudo">The hyperlink pseudo-class</a>
<td>4
<tr>
<td><code>E:link</code>
<td>an E element being the source anchor of a hyperlink
of which the target is not yet visited
<td><a href="#link">The link history pseudo-classes</a>
<td>1
<tr>
<td><code>E:visited</code>
<td>an E element being the source anchor of a hyperlink
of which the target is already visited
<td><a href="#link">The link history pseudo-classes</a>
<td>1
<tr>
<td><code>E:local-link</code>
<td>an E element being the source anchor of a hyperlink
of which the target is the current document
<td><a href="#local-pseudo">The local link pseudo-class</a>
<td>4
<tr>
<td><code>E:local-link(0)</code>
<td>an E element being the source anchor of a hyperlink
of which the target is within the current domain
<td><a href="#local-pseudo">The local link pseudo-class</a>
<td>4
<tr>
<td><code>E:target</code>
<td>an E element being the target of the referring URI
<td><a href="#target-pseudo">The target pseudo-class</a>
<td>3
<tr>
<td><code>E:scope</code>
<td>an E element being a designated contextual reference element
<td><a href="#scope-pseudo">The scope pseudo-class</a>
<td>3
<tbody>
<tr>
<td><code>E:current</code>
<td>an E element that is currently presented in a time-dimensional canvas
<td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
<td>4
<tr>
<td><code>E:current(<var>s</var>)</code>
<td>an E element that is the deepest <code>:current</code> element that
matches selector <var>s</var>
<td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
<td>4
<tr>
<td><code>E:past</code>
<td>an E element that is in the past in a time-dimensional canvas
<td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
<td>4
<tr>
<td><code>E:future</code>
<td>an E element that is in the future in a time-dimensional canvas
<td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
<td>4
<tbody>
<tr>
<td><code>E:active</code>
<td>an E element that is in an activated state
<td><a href="#useraction-pseudos">The user action pseudo-classes</a>
<td>1
<tr>
<td><code>E:hover</code>
<td>an E element that is under the cursor,
or that has a descendant under the cursor
<td><a href="#useraction-pseudos">The user action pseudo-classes</a>
<td>2
<tr>
<td><code>E:focus</code>
<td>an E element that has user input focus
<td><a href="#useraction-pseudos">The user action pseudo-classes</a>
<td>2
<tr>
<td><code>E:enabled<br>E:disabled</code>
<td>a user interface element E that is enabled or disabled, respectively
<td><a href="#enableddisabled">The :enabled and :disabled pseudo-classes</a>
<td>3
<tr>
<td><code>E:checked</code>
<td>a user interface element E that is checked/selected
(for instance a radio-button or checkbox)
<td><a href="#checked">The selected-option pseudo-class</a>
<td>3
<tr>
<td><code>E:indeterminate</code>
<td>a user interface element E that is in an
indeterminate state (neither checked nor unchecked)
<td><a href="#indeterminate">The indeterminate-value pseudo-class</a>
<td>4
<tr>
<td><code>E:default</code>
<td>a user interface element E that
<td><a href="#default-pseudo">The default option pseudo-class :default</a>
<td>3-UI/4
<tr>
<td><code>E:in-range</code><br><code>E:out-of-range</code>
<td>a user interface element E that
<td><a href="#range-pseudos">The validity pseudo-classes</a>
<td>3-UI/4
<tr>
<td><code>E:required</code><br><code>E:optional</code>
<td>a user interface element E that
<td><a href="#opt-pseudos">The optionality pseudo-classes</a>
<td>3-UI/4
<tr>
<td><code>E:read-only</code><br><code>E:read-write</code>
<td>a user interface element E that
<td><a href="#rw-pseudos">The mutability pseudo-classes</a>
<td>3-UI/4
<tbody>
<tr>
<td><code>E:root</code>
<td>an E element, root of the document
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:empty</code>
<td>an E element that has no children (not even text nodes)
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:first-child</code>
<td>an E element, first child of its parent
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>2
<tr>
<td><code>E:nth-child(<var>n</var>)</code>
<td>an E element, the <var>n</var>-th child of its parent
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:last-child</code>
<td>an E element, last child of its parent
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:nth-last-child(<var>n</var>)</code>
<td>an E element, the <var>n</var>-th child of its parent,
counting from the last one
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:only-child</code>
<td>an E element, only child of its parent
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:first-of-type</code>
<td>an E element, first sibling of its type
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:nth-of-type(<var>n</var>)</code>
<td>an E element, the <var>n</var>-th sibling of its type
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:last-of-type</code>
<td>an E element, last sibling of its type
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:nth-last-of-type(<var>n</var>)</code>
<td>an E element, the <var>n</var>-th sibling of its type,
counting from the last one
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:only-of-type</code>
<td>an E element, only sibling of its type
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>3
<tr>
<td><code>E:nth-match(<var>n</var> of <var>selector</var>)</code>
<td>an E element, the <var>n</var>-th sibling matching <var>selector</var>
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>4
<tr>
<td><code>E:nth-last-match(<var>n</var> of <var>selector</var>)</code>
<td>an E element, the <var>n</var>-th sibling matching <var>selector</var>,
counting from the last one
<td><a href="#structural-pseudos">Structural pseudo-classes</a>
<td>4
<tbody>
<tr>
<td><code>E:column(<var>selector</var>)</code>
<td>an E element that represents a cell in a grid/table
belonging to a column represented by an element that matches
<var>selector</var>
<td><a href="#table-pseudos">Grid-Structural pseudo-classes</a>
<td>4
<tr>
<td><code>E:nth-column(<var>n</var>)</code>
<td>an E element that represents a cell belonging to the
<var>n</var>th column in a grid/table
<td><a href="#table-pseudos">Grid-Structural pseudo-classes</a>
<td>4
<tr>
<td><code>E:nth-last-column(<var>n</var>)</code>
<td>an E element that represents a cell belonging to the
<var>n</var>th column in a grid/table, counting from the last one
<td><a href="#table-pseudos">Grid-Structural pseudo-classes</a>
<td>4
<tbody>
<tr>
<td><code>E F</code>
<td>an F element descendant of an E element
<td><a href="#descendant-combinators">Descendant combinator</a>
<td>1
<tr>
<td><code>E > F</code>
<td>an F element child of an E element
<td><a href="#child-combinators">Child combinator</a>
<td>2
<tr>
<td><code>E + F</code>
<td>an F element immediately preceded by an E element
<td><a href="#adjacent-sibling-combinators">Adjacent sibling combinator</a>
<td>2
<tr>
<td><code>E ~ F</code>
<td>an F element preceded by an E element
<td><a href="#general-sibling-combinators">General sibling combinator</a>
<td>3
<tr>
<td><code>E /foo/ F</code>
<td>an F element ID-referenced by an E element's <code>foo</code> attribute
<td><a href="#idref-combinators">Reference combinator</a>
<td>4
<tr>
<td><code>E! > F</code>
<td>an E element parent of an F element
<td><a href="#subject">Determining the subject of a selector</a> +
<a href="#child-combinators">Child combinator</a>
<td>4
</tbody></table>
<p>The meaning of each selector is derived from the table above by
prepending "matches" to the contents of each cell in the "Meaning"
column.</p>
<p class="note">Some Level 4 selectors (noted above as "3-UI") were
introduced in [[CSS3UI]].
<h2 id=syntax>
Selector Syntax and Structure</h2>
<h3 id=structure>
Structure and Terminology</h3>
<p>The term <dfn id=selector>selector</dfn> can refer to a <i>simple selector</i>,
<i>compound selector</i>, <i>complex selector</i>, or <i>selector list</i>.
<p>A <dfn id=complex>complex selector</dfn> is a chain of one
or more <a href="#compound">compound selectors</a>
separated by <a href="#combinators">combinators</a>.</p>
<p>A <dfn id=compound>compound selector</dfn>
is a chain of <a href="#simple">simple selectors</a>
that are not separated by a <a href="#combinators">combinator</a>. It
always begins with a <a href="#type-selectors">type selector</a> or a
(possibly implied) <a href="#universal-selector">universal
83AE
selector</a>.
No other type selector or universal selector is allowed in the sequence.</p>
<p>A <dfn id=simple><a
href="#simple">simple selector</a></dfn> is either a <a
href="#type-selectors">type selector</a>, <a
href="#universal-selector">universal selector</a>, <a
href="#attribute-selectors">attribute selector</a>, <a
href="#class-html">class selector</a>, <a
href="#id-selectors">ID selector</a>, or <a
href="#pseudo-classes">pseudo-class</a>.</p>
<p><dfn id=combinator>Combinators</dfn> are punctuation that represent a particular
kind of relationship between the compound selectors on either side.
Combinators in Selectors level 4 include: whitespace, "greater-than
sign" (U+003E, <code>></code>), "plus sign" (U+002B,
<code>+</code>) and "tilde" (U+007E, <code>~</code>).
<a href="#whitespace">White space</a> may appear between a combinator
and the simple selectors around it.</p>
<p>An empty selector, containing no compound selector, is an
<a href="#invalid">invalid selector</a>.</p>
<h3 id=subject>
Determining the Subject of a Selector</h3>
<p>The elements of a document tree that are represented by a selector
are the <dfn>subjects</dfn> of the selector.
<p>By default, the subjects of a selector are the elements represented
by the last compound selector in the selector. Thus a selector consisting
of a single compound selector represents any element satisfying its
requirements. Prepending another compound selector and a combinator
to a sequence imposes additional matching constraints, so the subjects of
the selector are always a subset of the elements represented by the last
compound selector.</p>
<p>The subject of the selector can be explicitly identified by appending
an exclamation mark (!) to one of the compound selectors in a selector.
Although the element structure that the selector represents is the same
with or without the exclamation mark, indicating the subject in this way
can change which compound selector represents the subject in that structure.
<div class="example">
<p>For example, the following selector represents a list item <code>LI</code>
unique child of an ordered list <code>OL</code>:
<pre>OL > LI:only-child</pre>
<p>However the following one represents an ordered list <code>OL</code>
having a unique child, that child being a <code>LI</code>:
<pre>OL! > LI:only-child</pre>
<p>The tree structures represented by these two selectors are the same,
but the subjects of the selectors are not.
</div>
<h3 id=pseudo-classes>
Pseudo-classes</h3>
<p>The pseudo-class concept is introduced to permit selection based on
information that lies outside of the document tree or that cannot be
expressed using the other simple selectors.</p>
<p>A pseudo-class always consists of a "colon"
(<code>:</code>) followed by the name of the pseudo-class and, for
functional pseudo-classes, by a value between parentheses.
White space is optionally allowed between the parentheses and the
argument, but not between the pseudo-class name and the parentheses.</p>
<p>Pseudo-classes are allowed in all compound selectors
contained in a selector. Pseudo-classes are allowed anywhere in a
compound selector after the leading type selector or
(possibly omitted) universal selector. Pseudo-class names are
case-insensitive. Some pseudo-classes are mutually exclusive (such
that a compound selector containing them, while valid, will never
match anything), while others can apply simultaneously to the same
element. Pseudo-classes may be dynamic, in the sense that an element
can acquire or lose a pseudo-class while a user interacts with the
document.</p>
<p>Dynamic pseudo-classes classify elements on characteristics other
than their name, attributes, or content, in principle characteristics
that cannot be deduced from the document tree.</p>
<p>Dynamic pseudo-classes do not appear in the document source or
document tree.</p>
<h3 id=pseudo-elements>Pseudo-elements</h3>
<p>Pseudo-elements create abstractions about the document tree beyond
those specified by the document language. For instance, document
languages do not offer mechanisms to access the first letter or first
line of an element's content. Pseudo-elements allow authors to refer
to this otherwise inaccessible information. Pseudo-elements may also
provide authors a way to refer to content that does not exist in the
source document (e.g., the <code>::before</code> and
<code>::after</code> pseudo-elements give access to generated
content in CSS [[CSS21]]).</p>
<p>A pseudo-element is made of two colons (<code>::</code>) followed
by the name of the pseudo-element.</p>
<p>This <code>::</code> notation was chosen in order to establish a
discrimination between pseudo-classes (which subclass existing elements)
and pseudo-elements (which are elements not represented in the document
tree). However, for compatibility with existing style sheets, user
agents must also accept the previous one-colon notation for
pseudo-elements introduced in CSS levels 1 and 2 (namely,
<code>:first-line</code>, <code>:first-letter</code>,
<code>:before</code> and <code>:after</code>). This compatibility
notation is not allowed any other pseudo-elements.</p>
<p>Only one pseudo-element may appear per selector, and only if the
subject of the selector is the last compound selector in the selector.
If present the pseudo-element must appear after the compound selector
that represents the <a href="#subject">subjects</a> of the selector.
<p class="note">A future version of this specification may allow
multiple pseudo-elements per selector.</p>
<p>Syntactically, a pseudo-element may be followed by any combination
of the <a href="#useraction-pseudos">user action pseudo-classes</a>.
Whether these pseudo-classes can match on the pseudo-element depends
on the pseudo-class and pseudo-element's definition: unless
otherwise-specified, none of these pseudo-classes will match on the
pseudo-element.
<div class="example">
<p>For example, the <code>:hover</code> pseudo-class specifies that it
can apply to any pseudo-element, i.e. <code>::first-line:hover</code>
will match when the first line is hovered.
However, since neither <code>:focus</code> nor <code>::first-line</code>
define that <code>:focus</code> can apply to <code>::first-line</code>,
the selector <code>::first-line:focus</code> will never match anything.
</div>
<h3 id=case-sensitive>
Characters and case sensitivity</h3>
<p>All Selectors syntax is case-insensitive within the ASCII range
(i.e. [a-z] and [A-Z] are equivalent), except for parts that are
not under the control of Selectors. The case sensitivity of document
language element names, attribute names, and attribute values in
selectors depends on the document language. For example, in HTML,
element names are case-insensitive, but in XML, they are
case-sensitive. Case sensitivity of namespace prefixes is defined in
[[!CSS3NAMESPACE]].</p>
<p><dfn id=whitespace>White space</dfn> in Selectors consists of the
characters SPACE (U+0020), TAB (U+0009), LINE FEED (U+000A),
CARRIAGE RETURN (U+000D), and FORM FEED (U+000C) can occur in whitespace.
Other space-like characters, such as EM SPACE (U+2003) and
IDEOGRAPHIC SPACE (U+3000), are never part of white space.
<p>Characters in Selectors can be escaped with a backslash according
to the same <a href="http://www.w3.org/TR/CSS21/syndata.html#characters">escaping
rules</a> as CSS. [[!CSS21]].</p>
<h3 id="namespaces">
Namespaces</h3>
<p>Certain selectors support namespace prefixes. The
mechanism by which namespace prefixes are <dfn id=nsdecl>declared</dfn> should
be specified by the language that uses Selectors. If the language does
not specify a namespace prefix declaration mechanism, then no prefixes
are declared. In CSS, namespace prefixes are declared with the
<a href="http://www.w3.org/TR/css3-namespace/#declaration"><code>@namespace</code></a>
rule. [[!CSS3NAMESPACE]]</p>
<h3 id=invalid>
Invalid Selectors and Error Handling</h3>
<p>Invalidity is caused by a parsing error, e.g. an unrecognized token or a token
which is not allowed at the current parsing point.
<p>User agents must observe the rules for handling parsing errors:
<ul>
<li>a simple selector containing an <a href="#namespaces">undeclared namespace prefix</a> is invalid</li>
<li>a selector containing an invalid simple selector, an invalid combinator
or an invalid token is invalid. </li>
<li>a selector list containing an invalid selector is invalid.</li>
</ul>
<p>An invalid selector represents nothing.
<p class="issue">It's been <a href="http://lists.w3.org/Archives/Public/www-style/2010Nov/0203.html">requested</a>
that the last rule be dropped in favor of Media Queries-style error-handling.</p>
<h2 id=logical-combination>
Logical Combinations</h2>
<h3 id=grouping>
Selector Lists</h3>
<p>A comma-separated list of selectors represents the union of all
elements selected by each of the individual selectors in the
<dfn>selector list</dfn>.
(A comma is U+002C.) For example, in CSS when several selectors share
the same declarations, they may be grouped into a comma-separated
list. White space may appear before and/or after the comma.</p>
<div class="example">
<p>CSS example:</p>
<p>In this example, we condense three rules with identical
declarations into one. Thus,</p>
<pre>h1 { font-family: sans-serif }
h2 { font-family: sans-serif }
h3 { font-family: sans-serif }</pre>
<p>is equivalent to:</p>
<pre>h1, h2, h3 { font-family: sans-serif }</pre>
</div>
<p><strong>Warning</strong>: the equivalence is true in this example
because all the selectors are valid selectors. If just one of these
selectors were invalid, the entire group of selectors would be
invalid. This would invalidate the rule for all three heading
elements, whereas in the former case only one of the three individual
heading rules would be invalidated.</p>
<div class="example">
<p>Invalid CSS example:</p>
<pre>h1 { font-family: sans-serif }
h2..foo { font-family: sans-serif }
h3 { font-family: sans-serif }</pre>
<p>is not equivalent to:</p>
<pre>h1, h2..foo, h3 { font-family: sans-serif }</pre>
<p>because the above selector (<code>h1, h2..foo, h3</code>)
is entirely invalid and the entire style rule is dropped. (When
the selectors are not grouped, only the rule for <code>h2..foo</code>
is dropped.)</p>
</div>
<h3 id=matches>
The Matches-Any Pseudo-class: '':matches()''</h3>
<p>The matches-any pseudo-class, <code>:matches(<var>X</var>)</code>,
is a functional notation taking a <a href="#grouping">selector list</a>
as its argument. It represents an element that is represented by its argument.
<p>In Selectors Level 4, only <a href="#compound">compound selectors</a>
are allowed within <code>:matches()</code>:
<a href="#combinator">combinators</a> are not allowed. Additionally,
<code>:matches()</code> may not be nested within itself or within
<code>:not()</code>: <code>:matches(:matches(...))</code> and
<code>:not(:matches(...))</code> are invalid.</p>
<p>Pseudo-elements cannot be represented by the matches-any pseudo-class;
they are not valid within <code>:matches()</code>.
<p>Default namespace declarations do not affect the subject of any selector
within a matches-any pseudo-class unless the argument is an explicit
universal selector or a type selector.</p>
<div class="example">
<p>For example, following selector matches any element that is being
hovered or focused, regardless of its namespace. In particular, it
is not limited to only matching elements in the default namespace
that are being hovered or focused.</p>
<pre>*|*:matches(:hover, :focus)</pre>
<p>The following selector, however, represents only hovered or focused
elements that are in the default namespace, because it uses an explicit
universal selector within the <code>:matches()</code> notation:</p>
<pre>*|*:matches(*:hover, *:focus)</pre>
</div>
<h3 id=negation>
The Negation Pseudo-class: '':not()''</h3>
<p>The negation pseudo-class, <code>:not(<var>X</var>)</code>, is a
functional notation taking a <a href="#grouping">selector list</a>
as an argument. It represents an element that is not represented
by its argument.
<p>In Selectors Level 4, only <a href="#compound">compound selectors</a>
are allowed within <code>:matches()</code>:
<a href="#combinator">combinators</a> are not allowed. Additionally,
negations may not be nested within itself or within <code>:matches()</code>:
<code>:not(:not(...))</code> and <code>:matches(:not(...))</code> are invalid.
<p class="note">In Selectors Level 3, only a single
<a href="#simple">simple selector</a> was allowed as the argument to
<code>:not()</code>.
<p>Pseudo-elements cannot be represented by the negation pseudo-class;
they are not valid within <code>:not()</code>.
<div class="example">
<p>For example, the following selector matches all <code>button</code>
elements in an HTML document that are not disabled.</p>
<pre>button:not([DISABLED])</pre>
<p>The following selector represents all but <code>FOO</code>
elements.</p>
<pre>*:not(FOO)</pre>
<p>The following compound selector represents all HTML elements
except links.</p>
<pre>html|*:not(:link):not(:visited)</pre>
</div>
<p>Default namespace declarations do not affect the subject of any selector
within a negation pseudo-class unless the argument is an explicit universal
selector or a type selector. (See <a href="#matches"><code>:matches()</code></a>
for examples.</p>
<p class="note"><strong>Note</strong>: the :not() pseudo allows
useless selectors to be written. For instance <code>:not(*|*)</code>,
which represents no element at all, or <code>foo:not(bar)</code>,
which is equivalent to <code>foo</code> but with a higher
specificity.</p>
<h2 id=elemental-selectors>
Elemental selectors</h2>
<h3 id=type-selectors>
Type selector</h3>
<p>A <dfn>type selector</dfn> is the name of a document language
element type written using the syntax of
<a href="http://www.w3.org/TR/css3-namespace/#css-qnames">CSS qualified
names</a> [[!CSS3NAMESPACE]]. A type selector represents
an instance of the element type in the document tree. </p>
<div class="example">
<p>Example:</p>
<p>The following selector represents an <code>h1</code> element in the document tree:</p>
<pre>h1</pre>
</div>
<h4 id=typenmsp>
Type selectors and namespaces</h4>
<p>Type selectors allow an optional namespace component: a namespace
prefix that has been previously <a href="#nsdecl">declared</a> may be
prepended to the element name separated by the namespace separator
"vertical bar" (U+007C, <code>|</code>). (See, e.g., [[XML-NAMES]] for the use of namespaces in
XML.)</p>
<p>The namespace component may be left empty (no prefix before the
namespace separator) to indicate that the selector is only to
represent elements with no namespace.</p>
<p>An asterisk may be used for the namespace prefix, indicating that
the selector represents elements in any namespace (including elements
with no namespace).</p>
<p>Element type selectors that have no namespace component (no
namespace separator) represent elements without regard to the
element's namespace (equivalent to "<code>*|</code>") unless a default
namespace has been <a href="#nsdecl">declared</a> for namespaced selectors (e.g. in CSS, in
the style sheet). If a default namespace has been declared, such
selectors will represent only elements in the default namespace.</p>
<p>A type selector containing a namespace prefix that has not been
previously <a href="#nsdecl">declared</a> for namespaced selectors is
an <a href="#conformance">invalid</a> selector.</p>
<p>In a namespace-aware client, the name part of element type
selectors (the part after the namespace separator, if it is present)
will only match against the <a
href="http://www.w3.org/TR/REC-xml-names/#NT-LocalPart">local part</a>
of the element's <a
href="http://www.w3.org/TR/REC-xml-names/#ns-qualnames">qualified
name</a>.</p>
<p>In summary:</p>
<dl>
<dt><code>ns|E</code></dt>
<dd>elements with name E in namespace ns</dd>
<dt><code>*|E</code></dt>
<dd>elements with name E in any namespace, including those without a
namespace</dd>
<dt><code>|E</code></dt>
<dd>elements with name E without a namespace</dd>
<dt><code>E</code></dt>
<dd>if no default namespace has been <a href="#nsdecl">declared</a> for selectors, this is
equivalent to *|E. Otherwise it is equivalent to ns|E where ns is
the default namespace.</dd>
</dl>
<div class="example">
<p>CSS examples:</p>
<pre>@namespace foo url(http://www.example.com);
foo|h1 { color: blue } /* first rule */
foo|* { color: yellow } /* second rule */
|h1 { color: red } /* ...*/
*|h1 { color: green }
h1 { color: green }</pre>
<p>The first rule (not counting the <code>@namespace</code> at-rule)
will match only <code>h1</code> elements in the
"http://www.example.com" namespace.</p>
<p>The second rule will match all elements in the
"http://www.example.com" namespace.</p>
<p>The third rule will match only <code>h1</code> elements with
no namespace.</p>
<p>The fourth rule will match <code>h1</code> elements in any
namespace (including those without any namespace).</p>
<p>The last rule is equivalent to the fourth rule because no default
namespace has been defined.</p>
</div>
<h3 id=universal-selector>
Universal selector </h3>
<p>The <dfn>universal selector</dfn>, written as a
<a href="http://www.w3.org/TR/css3-namespace/#css-qnames">CSS qualified
name</a> [[!CSS3NAMESPACE]] with an asterisk (<code>*</code>
U+002A) as the local name, represents the qualified name of any element
type. It represents any single element in the document tree in any
namespace (including those without a namespace) if no default
namespace has been specified for selectors. If a default namespace has
been specified, see <a href="#univnmsp">Universal selector and
Namespaces</a> below.</p>
<p>If a universal selector represented by <code>*</code> (i.e. without a
namespace prefix) is not the only component of a <a href="#compound">compound
selector</a> or is immediately followed by a
<a href="#pseudo-elements">pseudo-element</a>, then the <code>*</code> may
be omitted and the universal selector's presence implied.</p>
<div class="example">
<p>Examples:</p>
<ul>
<li><code>*[hreflang|=en]</code> and <code>[hreflang|=en]</code> are equivalent,</li>
<li><code>*.warning</code> and <code>.warning</code> are equivalent,</li>
<li><code>*#myid</code> and <code>#myid</code> are equivalent.</li>
</ul>
</div>
<p class="note"><strong>Note:</strong> it is recommended that the
<code>*</code> not be omitted, because it decreases the potential
confusion between, for example,
<code style="white-space: nowrap">div :first-child</code>
and <code style="white-space: nowrap">div:first-child</code>.
Here, <code style="white-space: nowrap">div *:first-child</code>
is more readable.</p>
<h4 id=univnmsp>
Universal selector and namespaces</h4>
<p>The universal selector allows an optional namespace component. It
is used as follows:</p>
<dl>
<dt><code>ns|*</code></dt>
<dd>all elements in namespace ns</dd>
<dt><code>*|*</code></dt>
<dd>all elements</dd>
<dt><code>|*</code></dt>
<dd>all elements without a namespace</dd>
<dt><code>*</code></dt>
<dd>if no default namespace has been specified, this is equivalent to *|*.
Otherwise it is equivalent to ns|* where ns is the default namespace.</dd>
</dl>
<p>A universal selector containing a namespace prefix that has not
been previously <a href="#nsdecl">declared</a> is an <a
href="#conformance">invalid</a> selector.</p>
<h2 id=attribute-selectors>
Attribute selectors</h2>
<p>Selectors allow the representation of an element's attributes. When
a selector is used as an expression to match against an element,
attribute selectors must be considered to match an element if that
element has an attribute that matches the attribute represented by the
attribute selector.</p>
<p class="issue">Add comma-separated syntax for
<a href="http://lists.w3.org/Archives/Public/www-style/2011Mar/0215.html">multiple-value matching</a>?
e.g. [rel ~= next, prev, up, first, last]
<h3 id=attribute-representation>