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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]/ED-[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 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>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.
<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>The following features are at-risk and may be dropped during the CR period
if there is not sufficient implementer interest:
the reference combinator,
the column combinator,
the '':invalid-drop'' and '':valid-drop'' pseudo-classes.
<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>
A selector is a boolean predicate
that takes an element in a tree structure
and tests whether the element matches the selector or not.
<p>
These expressions may be used for many things:
<ul>
<li>
directly on an element to test whether it matches some criteria,
such as in the <code>Element.matches()</code> function defined in [[SELECTORS-API2]]
<li>
applied to an entire tree of elements
to filter it into a set of elements that match the criteria,
such as in the <code>document.findAll()</code> function defined in [[SELECTORS-API2]]
or the selector of a CSS style rule.
<li>
used "in reverse" to generate markup that would match a given selector,
such as in <a href="http://haml.info/">HAML</a> or <a href="http://en.wikipedia.org/wiki/Zen_Coding">Zen Coding</a>
</ul>
<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 of
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>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>Selectors may range from simple element names to rich contextual
representations.
<p>The following table summarizes the Selector syntax:
<table class="data">
<col class="pattern">
<col class="meaning">
<col class="section">
<col class="level">
<thead>
<tr>
<th>Pattern
<th>Represents
<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 (tag name) 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 <i>compound selector</i> <var>s1</var>
or <i>compound selector</i> <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 <i>compound selector</i> <var>s1</var>
and/or <i>compound selector</i> <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(zh, *-hant)</code>
<td>an element of type E tagged as being either in Chinese
(any dialect or writing system)
or othewise written with traditional Chinese characters
<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>4
<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&g
1714
t;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">Next-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">Following-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 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>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 <i>compound selectors</i>
separated by <i>combinators</i>.
<p>A <dfn id="compound">compound selector</dfn>
is a chain of <i>simple selectors</i>
that are not separated by a <i>combinator</i>. It
always begins with a <i>type selector</i> or a
(possibly implied) <i>universal selector</i>.
No other type selector or universal selector is allowed in the sequence.
<p>
A <dfn id="simple">simple selector</dfn> is either a
<i>type selector</i>,
<i>universal selector</i>,
<i>attribute selector</i>,
<i>class selector</i>,
<i>ID selector</i>,
or <i>pseudo-class</i>.
<p>A <dfn>combinator</dfn> is punctuation that represents a particular
kind of relationship between the <i>compound selectors</i> 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>An empty selector, containing no <i>compound selector</i>, is an
<a href="#invalid">invalid selector</a>.
<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 <i>compound selector</i> in the selector.
Thus a selector consisting of a single <i>compound selector</i>
represents any element satisfying its requirements
Prepending another <i>compound selector</i> 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 <i>compound selector</i>.
<p>The subject of the selector can be explicitly identified
by prepending an exclamation mark (!)
to one of the <i>compound selectors</i> 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 <i>compound selector</i> represents the subject in that structure.
<p class="issue">
Should the exclamation mark be prepended or appended to the subject? Or both?
Or prepend two, to avoid the "! = not" issue?
<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="scoping">
Scoped Selectors</h3>
<p>Some host applications may choose to <dfn>scope</dfn> selectors
to a particular subtree of the document.
The root of the scoping subtree is called the <dfn>scoping element</dfn>,
and is in-scope.
When scoped selectors are used, it forms the <i>contextual reference element set</i>
and matches the <code>:scope</code> pseudo-class.
<p>There are three methods of scoping selectors:
<dl>
<dt><dfn>scope-contained</dfn> selectors
<dd>
With this method of scoping, selectors match as if
the <i>scoping element</i> were the root of the document:
all <i>compound selectors</i> must match elements within the scope.
(The <code>:root</code> pseudo-class, however,
still only matches the actual root of the document.)
<dt><dfn>scope-filtered</dfn> selectors
<dd>
With this method of scoping, a selector matches if
the <i>subject</i> of the selector is within the scope,
even if other components of the selector are outside the scope.
<dt><dfn>scope-relative</dfn> selectors
<dd>
With this method of scoping,
"<code>:scope</code> " (the <code>:scope</code> pseudo-class followed by a space)
is implied at the beginning of each <i>complex selector</i>,
allowing them to begin syntactically with a <i>combinator</i>.
The scoping element matches this implied <code>:scope</code> selector,
but does not limit which elements match.
</dl>
<div class='example'>
<p>
For example,
the <code>element.querySelector()</code> function defined in [[SELECTORS-API2]]
allows the author to define a <i>scope-filtered</i> selector,
while the similar <code>element.find</code> function defined in the same spec
uses <i>scope-relative</i> selectors.
<p>
On the other hand, the selectors within an [[HTML5]] scoped stylesheet
define <i>scope-contained</i> selectors.
</div>
<h3 id="pseudo-classes">
Pseudo-classes</h3>
<p>The <dfn>pseudo-class</dfn> 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>A <i>pseudo-class</i> always consists of a "colon"
(<code>:</code>) followed by the name of the <i>pseudo-class</i> and, for
<dfn>functional pseudo-classes</dfn>, 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><i>Pseudo-classes</i> are allowed in all <i>compound selectors</i> contained in a selector.
<i>Pseudo-classes</i> are allowed anywhere in a <i>compound selector</i>
after the leading type selector or (possibly omitted) universal selector.
<i>Pseudo-class</i> names are case-insensitive.
Some <i>pseudo-classes</i> are mutually exclusive
(such that a <i>compound selector</i> containing them, while valid, will never match anything),
while others can apply simultaneously to the same element.
<i>Pseudo-classes</i> may be dynamic,
in the sense that an element can acquire or lose a pseudo-class
while a user interacts with the document.
<p>Dynamic <i>pseudo-classes</i> classify elements on characteristics
other than their name, attributes, or content,
but rather on characteristics that cannot be deduced from the document tree.
They do not appear in or modify the document source or document tree.
<h3 id="pseudo-elements">Pseudo-elements</h3>
<p><dfn id='pseudo-element' title="pseudo-element">Pseudo-elements</dfn> 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.
<i>Pseudo-elements</i> allow authors to refer
to this otherwise inaccessible information.
<i>Pseudo-elements</i> 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> <i>pseudo-elements</i>
give access to generated content in CSS [[CSS21]]).
<p>A <i>pseudo-element</i> is made of two colons (<code>::</code>)
followed by the name of the <i>pseudo-element</i>.
<p>This <code>::</code> notation was chosen in order to establish
a discrimination between <i>pseudo-classes</i> (which subclass existing elements)
and <i>pseudo-elements</i> (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 <i>pseudo-elements</i> 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 <i>pseudo-elements</i>.
<p class="note">A future version of this specification may allow
multiple pseudo-elements per selector.
<p>Syntactically, a <i>pseudo-element</i> immediately follows
the <i>compound selector
1714
</i> representing its <dfn>originating element</dfn>,
i.e. the element to which it is associated.
Unless otherwise overridden by the definition of the <i>pseudo-element</i>:
<ul>
<li>only one <i>pseudo-element</i> may appear per <i>complex selector</i>
<li>the <i>pseudo-element</i> must appear after the <i>compound selector</i>
that represents the <a href="#subject">subjects</a> of the selector
<li>the <i>pseudo-element</i> may appear only if the subject of the selector
is the last <i>compound selector</i> in the selector.
</ul>
<p>A <i>pseudo-element</i> may be immediately followed
by any combination of the <a href="#useraction-pseudos">user action pseudo-classes</a>,
in which case the <i>pseudo-element</i> is represented only when it is in the corresponding state.
Whether these pseudo-classes can match on the <i>pseudo-element</i>
depends on the <i>pseudo-class</i> and <i>pseudo-element</i>”s definitions:
unless otherwise-specified, none of these <i>pseudo-classes</i>
will match on the <i>pseudo-element</i>.
<div class="example">
<p>For example, the <code>:hover</code> pseudo-class
8096
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>
<p>The host language defines which pseudo-elements exist and their meaning.
For CSS, [[CSS21]] defines the ::before, ::after, ::first-line and ::first-letter pseudo-elements.
<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><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]].
<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]]
<h3 id="invalid">
Invalid Selectors and Error Handling</h3>
<p>User agents must observe the rules for handling <dfn title="invalid selector|invalid">invalid selectors</dfn>:
<ul>
<li>a parsing error in a selector,
e.g. an unrecognized token or a token which is not allowed at the current parsing point,
causes that selector to be invalid
<li>a simple selector containing an <a href="#namespaces">undeclared namespace prefix</a> is invalid
<li>a selector containing an invalid simple selector, an invalid combinator
or an invalid token is invalid.
<li>a selector list containing an invalid selector is invalid.
</ul>
<p>An <i>invalid selector</i> 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.
<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.
<div class="example">
<p>CSS example:
<p>In this example, we condense three rules with identical
declarations into one. Thus,
<pre><!--
-->h1 { font-family: sans-serif }
<!--
-->h2 { font-family: sans-serif }
<!--
-->h3 { font-family: sans-serif }</pre>
<p>is equivalent to:
<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 <i>selector list</i> 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.
<div class="example">
<p>Invalid CSS example:
<pre><!--
-->h1 { font-family: sans-serif }
<!--
-->h2..foo { font-family: sans-serif }
<!--
-->h3 { font-family: sans-serif }</pre>
<p>is not equivalent to:
<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.)
</div>
<h3 id="matches">
The Matches-Any Pseudo-class: <code>:matches()</code></h3>
<p>The matches-any pseudo-class, <dfn id='matches-pseudo'>:matches()</dfn>,
is a functional pseudo-class 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>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.
<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.
<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:
<pre>*|*:matches(*:hover, *:focus)</pre>
</div>
<h3 id="negation">
The Negation Pseudo-class: <code>:not()</code></h3>
<p>The negation pseudo-class, <dfn id='negation-pseudo'>:not()</dfn>, is a
functional pseudo-class 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>:not()</code>:
<a href="#combinator">combinators</a> are not allowed. Additionally,
a negation 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.
<pre>button:not([DISABLED])</pre>
<p>The following selector represents all but <code>FOO</code>
elements.
<pre>*:not(FOO)</pre>
<p>The following compound selector represents all HTML elements
except links.
<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 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.
<h2 id="elemental-selectors">
Elemental selectors</h2>
<h3 id="type-selectors">
Type (tag name) 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.
<div class="example">
<p>Example:
<p>The following selector represents an <code>h1</code> element in the document tree:
<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>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>An asterisk may be used for the namespace prefix, indicating that
the selector represents elements in any namespace (including elements
with no namespace).
<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>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>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>In summary:
<dl>
<dt><code>ns|E</code>
<dd>elements with name E in namespace ns
<dt><code>*|E</code>
<dd>elements with name E in any namespace, including those without a
namespace
<dt><code>|E</code>
<dd>elements with name E without a namespace
<dt><code>E</code>
<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.
</dl>
<div class="example">
<p>CSS examples:
<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>The second rule will match all elements in the
"http://www.example.com" namespace.
<p>The third rule will match only <code>h1</code> elements with