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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html lang=en>
<head>
<title>Selectors Level 4</title>
<link href=default.css rel=stylesheet type="text/css">
<link href="http://www.w3.org/StyleSheets/TR/W3C-ED.css" rel=stylesheet
type="text/css">
<body>
<div class=head> <!--begin-logo-->
<p><a href="http://www.w3.org/"><img alt=W3C height=48
src="http://www.w3.org/Icons/w3c_home" width=72></a> <!--end-logo-->
<h1 id=title>Selectors Level 4</h1>
<h2 class="no-num no-toc" id=longstatus-date>Editor's Draft 4 June 2011</h2>
<dl>
<dt>This version:
<dd> <!-- <a href="http://www.w3.org/TR/2011/ED-selectors4-20110604/">
http://www.w3.org/TR/2011/PR-selectors4-20110604</a> -->
<a href="http://dev.w3.org/csswg/selectors4">
http://dev.w3.org/csswg/selectors4</a>
<dt>Latest version:
<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/css3-selectors/">
http://www.w3.org/TR/css3-selectors/</a>
<dt><a name=editors-list></a>Editors:
<dd class=vcard><a class="url fn" href="http://www.tantek.com/"
lang=tr>Tantek Çelik</a> (Invited Expert)
<dd class=vcard><a class="url fn"
href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a>
(Invited Expert)
<dd class=vcard><span class=fn>Daniel Glazman</span> (Disruptive
Innovations SARL)
</dl>
<!--begin-copyright-->
<p class=copyright><a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright"
rel=license>Copyright</a> © 2011 <a
href="http://www.w3.org/"><acronym title="World Wide Web
Consortium">W3C</acronym></a><sup>®</sup> (<a
href="http://www.csail.mit.edu/"><acronym title="Massachusetts Institute
of Technology">MIT</acronym></a>, <a href="http://www.ercim.eu/"><acronym
title="European Research Consortium for Informatics and
Mathematics">ERCIM</acronym></a>, <a
href="http://www.keio.ac.jp/">Keio</a>), All Rights Reserved. W3C <a
href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>,
<a
href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a>
and <a
href="http://www.w3.org/Consortium/Legal/copyright-documents">document
use</a> rules apply.</p>
<!--end-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 <acronym title="Cascading Style
Sheets">CSS</acronym> (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
[[!CSS3SELECT]], 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>
<!--begin-status-->
<p>This is a public copy of the editors' draft. It is provided for
discussion only and may change at any moment. Its publication here does
not imply endorsement of its contents by W3C. Don't cite this document
other than as work in progress.
<p>The (<a
href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public
mailing list <a
href="mailto:www-style@w3.org?Subject=%5Bselectors4%5D%20PUT%20SUBJECT%20HERE">
www-style@w3.org</a> (see <a
href="http://www.w3.org/Mail/Request">instructions</a>) is preferred for
discussion of this specification. When sending e-mail, please put the text
“selectors4” in the subject, preferably like this:
“[<!---->selectors4<!---->] <em>…summary of
comment…</em>”
<p>This document was produced by the <a href="/Style/CSS/members">CSS
Working Group</a> (part of the <a href="/Style/">Style Activity</a>).
<p>This document was produced by a group operating under the <a
href="/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent
Policy</a>. W3C maintains a <a href="/2004/01/pp-impl/32061/status"
rel=disclosure>public list of any patent disclosures</a> made in
connection with the deliverables of the group; that page also includes
instructions for disclosing a patent. An individual who has actual
knowledge of a patent which the individual believes contains <a
href="/Consortium/Patent-Policy-20040205/#def-essential">Essential
Claim(s)</a> must disclose the information in accordance with <a
href="/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the
W3C Patent Policy</a>.</p>
<!--end-status-->
<h2 class="no-num no-toc" id=contents>Table of contents</h2>
<!--begin-toc-->
<ul class=toc>
<li><a href="#context"><span class=secno>1. </span> Introduction</a>
<ul class=toc>
<li><a href="#context"><span class=secno>1.1. </span>Background</a>
<li><a href="#placement"><span class=secno>1.2. </span>Module
Interactions</a>
<li><a href="#conventions"><span class=secno>1.3. </span>Document
Conventions</a>
</ul>
<li><a href="#overview"><span class=secno>2. </span> Selectors
Overview</a>
<li><a href="#syntax"><span class=secno>3. </span> Selector Syntax</a>
<ul class=toc>
<li><a href="#structure"><span class=secno>3.1. </span> Structure and
Terminology </a>
<li><a href="#subject"><span class=secno>3.2. </span> Subject of a
Selector </a>
<li><a href="#case-sensitive"><span class=secno>3.3. </span> Characters
and case sensitivity</a>
<li><a href="#namespaces"><span class=secno>3.4. </span> Namespaces</a>
<li><a href="#invalid"><span class=secno>3.5. </span> Invalid Selectors
and Error Handling</a>
</ul>
<li><a href="#logical-combination"><span class=secno>4. </span> Logical
Combinations</a>
<ul class=toc>
<li><a href="#grouping"><span class=secno>4.1. </span> Selector Groups
</a>
<li><a href="#matches"><span class=secno>4.2. </span> The Matches-Any
Pseudo-class: ‘<code class=css>:matches()</code>’</a>
<li><a href="#negation"><span class=secno>4.3. </span> The Negation
Pseudo-class: ‘<code class=css>:not()</code>’</a>
</ul>
<li><a href="#elemental-selectors"><span class=secno>5. </span> Elemental
selectors</a>
<ul class=toc>
<li><a href="#type-selectors"><span class=secno>5.1. </span> Type
selector</a>
<ul class=toc>
<li><a href="#typenmsp"><span class=secno>5.1.1. </span> Type
selectors and namespaces</a>
</ul>
<li><a href="#universal-selector"><span class=secno>5.2. </span>
Universal selector </a>
<ul class=toc>
<li><a href="#univnmsp"><span class=secno>5.2.1. </span> Universal
selector and namespaces</a>
</ul>
</ul>
<li><a href="#attribute-selectors"><span class=secno>6. </span> Attribute
selectors</a>
<ul class=toc>
<li><a href="#attribute-representation"><span class=secno>6.1. </span>
Attribute presence and value selectors</a>
<li><a href="#attribute-substrings"><span class=secno>6.2. </span>
Substring matching attribute selectors</a>
<li><a href="#attrnmsp"><span class=secno>6.3. </span> Attribute
selectors and namespaces</a>
<li><a href="#def-values"><span class=secno>6.4. </span> Default
attribute values in DTDs</a>
<li><a href="#class-html"><span class=secno>6.5. </span> Class
selectors</a>
<li><a href="#id-selectors"><span class=secno>6.6. </span> ID
selectors</a>
</ul>
<li><a href="#pseudo-classes"><span class=secno>7. </span> Pseudo-classes
Syntax and Overview</a>
<li><a href="#dynamic-pseudos"><span class=secno>8. </span> Dynamic
pseudo-classes</a>
<ul class=toc>
<li><a href="#link"><span class=secno>8.1. </span> The link
pseudo-classes: :link and :visited</a>
<li><a href="#useraction-pseudos"><span class=secno>8.2. </span> The
user action pseudo-classes :hover, :active, and :focus</a>
<li><a href="#target-pseudo"><span class=secno>8.3. </span> The target
pseudo-class :target</a>
</ul>
<li><a href="#linguistic-pseudos"><span class=secno>9. </span> Linguistic
Pseudo-classes</a>
<ul class=toc>
<li><a href="#dir-pseudo"><span class=secno>9.1. </span> The
directionality pseudo-class <code>:dir()</code></a>
<li><a href="#lang-pseudo"><span class=secno>9.2. </span> The language
pseudo-class <code>:lang</code></a>
</ul>
<li><a href="#UIstates"><span class=secno>10. </span> The UI element
states pseudo-classes</a>
<ul class=toc>
<li><a href="#enableddisabled"><span class=secno>10.1. </span> The
:enabled and :disabled pseudo-classes</a>
<li><a href="#checked"><span class=secno>10.2. </span> The :checked
pseudo-class</a>
<li><a href="#indeterminate"><span class=secno>10.3. </span> The
:indeterminate pseudo-class</a>
</ul>
<li><a href="#structural-pseudos"><span class=secno>11. </span> Structural
pseudo-classes</a>
<ul class=toc>
<li><a href="#root-pseudo"><span class=secno>11.1. </span> :root
pseudo-class</a>
<li><a href="#nth-child-pseudo"><span class=secno>11.2. </span>
:nth-child() pseudo-class</a>
<li><a href="#nth-last-child-pseudo"><span class=secno>11.3. </span>
:nth-last-child() pseudo-class</a>
<li><a href="#nth-of-type-pseudo"><span class=secno>11.4. </span>
:nth-of-type() pseudo-class</a>
<li><a href="#nth-last-of-type-pseudo"><span class=secno>11.5. </span>
:nth-last-of-type() pseudo-class</a>
<li><a href="#first-child-pseudo"><span class=secno>11.6. </span>
:first-child pseudo-class</a>
<li><a href="#last-child-pseudo"><span class=secno>11.7. </span>
:last-child pseudo-class</a>
<li><a href="#first-of-type-pseudo"><span class=secno>11.8. </span>
:first-of-type pseudo-class</a>
<li><a href="#last-of-type-pseudo"><span class=secno>11.9. </span>
:last-of-type pseudo-class</a>
<li><a href="#only-child-pseudo"><span class=secno>11.10. </span>
:only-child pseudo-class</a>
<li><a href="#only-of-type-pseudo"><span class=secno>11.11. </span>
:only-of-type pseudo-class</a>
<li><a href="#empty-pseudo"><span class=secno>11.12. </span> :empty
pseudo-class</a>
</ul>
<li><a href="#combinators"><span class=secno>12. </span> Combinators</a>
<ul class=toc>
<li><a href="#descendant-combinators"><span class=secno>12.1. </span>
Descendant combinator</a>
<li><a href="#child-combinators"><span class=secno>12.2. </span> Child
combinators</a>
<li><a href="#sibling-combinators"><span class=secno>12.3. </span>
Sibling combinators</a>
<ul class=toc>
<li><a href="#adjacent-sibling-combinators"><span class=secno>12.3.1.
</span> Adjacent sibling combinator</a>
<li><a href="#general-sibling-combinators"><span class=secno>12.3.2.
</span> General sibling combinator</a>
</ul>
</ul>
<li><a href="#specificity"><span class=secno>13. </span> Calculating a
selector's specificity</a>
<li><a href="#grammar"><span class=secno>14. </span> The grammar of
Selectors</a>
<ul class=toc>
<li><a href="#grammar"><span class=secno>14.1. </span> Grammar</a>
<li><a href="#lex"><span class=secno>14.2. </span> Lexical scanner</a>
</ul>
<li><a href="#profiling"><span class=secno>15. </span>Profiles</a>
<li><a href="#conformance"><span class=secno>16. </span> Conformance</a>
<ul class=toc>
<li><a href="#conformance-classes"><span class=secno>16.1. </span>
Conformance Classes</a>
<li><a href="#partial"><span class=secno>16.2. </span> Partial
Implementations</a>
<li><a href="#experimental"><span class=secno>16.3. </span> Experimental
Implementations</a>
</ul>
<li><a href="#acknowledgements"><span class=secno>17. </span>
Acknowledgements</a>
<li><a href="#references"><span class=secno>18. </span> References</a>
<ul class=toc>
<li><a href="#normative-references"><span class=secno>18.1. </span>
Normative References</a>
<li><a href="#informative-references"><span class=secno>18.2. </span>
Informative References</a>
</ul>
</ul>
<!--end-toc-->
<h2 id=context><span class=secno>1. </span> Introduction</h2>
<h3 id=context><span class=secno>1.1. </span>Background</h3>
<p><em>This section is not normative.</em>
<p>Selectors define the following function:
<pre>expression ∗ element → boolean</pre>
<p>That is, given an element and a selector, this specification defines
whether that element matches the selector.
<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. <acronym title="Simple
Tree Transformation Sheets">STTS</acronym> (Simple Tree Transformation
Sheets), a language for transforming XML trees, uses this mechanism. <a
href="#STTS3" rel=biblioentry>[STTS3]<!--{{STTS3}}--></a>
<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.
<h3 id=placement><span class=secno>1.2. </span>Module Interactions</h3>
<p>This module replaces the definitions for, and extends the set of
selectors (aside from pseudo-element selectors) defined for CSS in
[[CSS3SELECT]] and <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{CSS21}}--></a>.
<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 they are defined in a separate Pseudo-Element
Selectors Level 4 module.
<h3 id=conventions><span class=secno>1.3. </span>Document Conventions</h3>
<p>Conformance requirements are expressed with a combination of descriptive
assertions and RFC 2119 terminology. The key words “MUST”, “MUST
NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”,
“SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the
normative parts of this document are to be interpreted as described in RFC
2119. However, for readability, these words do not appear in all uppercase
letters in this specification.
<p>All of the text of this specification is normative except sections
explicitly marked as non-normative, examples, and notes. <a
href="#RFC2119" rel=biblioentry>[RFC2119]<!--{{!RFC2119}}--></a>
<p>Examples in this specification are introduced with the words “for
example” or are set apart from the normative text with
<code>class="example"</code>, like this:
<div class=example>
<p>This is an example of an informative example.</p>
</div>
<p>Informative notes begin with the word “Note” and are set apart from
the normative text with <code>class="note"</code>, like this:
<p class=note>Note, this is an informative note.
<h2 id=overview><span class=secno>2. </span> 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="selectorsReview data">
<thead>
<tr>
&
619F
lt;th class=pattern>Pattern
<th class=meaning>Meaning
<th class=described>Described in section
<th class=origin>First defined in CSS level
<tbody>
<tr>
<td class=pattern>*
<td class=meaning>any element
<td class=described><a href="#universal-selector">Universal selector</a>
<td class=origin>2
<tr>
<td class=pattern>E
<td class=meaning>an element of type E
<td class=described><a href="#type-selectors">Type selector</a>
<td class=origin>1
<tr>
<td class=pattern>E[foo]
<td class=meaning>an E element with a "foo" attribute
<td class=described><a href="#attribute-selectors">Attribute
selectors</a>
<td class=origin>2
<tr>
<td class=pattern>E[foo="bar"]
<td class=meaning>an E element whose "foo" attribute value is exactly
equal to "bar"
<td class=described><a href="#attribute-selectors">Attribute
selectors</a>
<td class=origin>2
<tr>
<td class=pattern>E[foo~="bar"]
<td class=meaning>an E element whose "foo" attribute value is a list of
whitespace-separated values, one of which is exactly equal to "bar"
<td class=described><a href="#attribute-selectors">Attribute
selectors</a>
<td class=origin>2
<tr>
<td class=pattern>E[foo^="bar"]
<td class=meaning>an E element whose "foo" attribute value begins
exactly with the string "bar"
<td class=described><a href="#attribute-selectors">Attribute
selectors</a>
<td class=origin>3
<tr>
<td class=pattern>E[foo$="bar"]
<td class=meaning>an E element whose "foo" attribute value ends exactly
with the string "bar"
<td class=described><a href="#attribute-selectors">Attribute
selectors</a>
<td class=origin>3
<tr>
<td class=pattern>E[foo*="bar"]
<td class=meaning>an E element whose "foo" attribute value contains the
substring "bar"
<td class=described><a href="#attribute-selectors">Attribute
selectors</a>
<td class=origin>3
<tr>
<td class=pattern>E[foo|="en"]
<td class=meaning>an E element whose "foo" attribute has a
hyphen-separated list of values beginning (from the left) with "en"
<td class=described><a href="#attribute-selectors">Attribute
selectors</a>
<td class=origin>2
<tr>
<td class=pattern>E:root
<td class=meaning>an E element, root of the document
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:nth-child(n)
<td class=meaning>an E element, the n-th child of its parent
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:nth-last-child(n)
<td class=meaning>an E element, the n-th child of its parent, counting
from the last one
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:nth-of-type(n)
<td class=meaning>an E element, the n-th sibling of its type
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:nth-last-of-type(n)
<td class=meaning>an E element, the n-th sibling of its type, counting
from the last one
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:first-child
<td class=meaning>an E element, first child of its parent
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>2
<tr>
<td class=pattern>E:last-child
<td class=meaning>an E element, last child of its parent
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:first-of-type
<td class=meaning>an E element, first sibling of its type
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:last-of-type
<td class=meaning>an E element, last sibling of its type
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:only-child
<td class=meaning>an E element, only child of its parent
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:only-of-type
<td class=meaning>an E element, only sibling of its type
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:empty
<td class=meaning>an E element that has no children (including text
nodes)
<td class=described><a href="#structural-pseudos">Structural
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:link<br>
E:visited
<td class=meaning>an E element being the source anchor of a hyperlink of
which the target is not yet visited (:link) or already visited
(:visited)
<td class=described><a href="#link">The link pseudo-classes</a>
<td class=origin>1
<tr>
<td class=pattern>E:active<br>
E:hover<br>
E:focus
<td class=meaning>an E element during certain user actions
<td class=described><a href="#useraction-pseudos">The user action
pseudo-classes</a>
<td class=origin>1 and 2
<tr>
<td class=pattern>E:target
<td class=meaning>an E element being the target of the referring URI
<td class=described><a href="#target-pseudo">The target pseudo-class</a>
<td class=origin>3
<tr>
<td class=pattern>E:lang(fr)
<td class=meaning>an element of type E in language "fr" (the document
language specifies how language is determined)
<td class=described><a href="#lang-pseudo">The :lang() pseudo-class</a>
<td class=origin>2
<tr>
<td class=pattern>E:enabled<br>
E:disabled
<td class=meaning>a user interface element E which is enabled or
disabled
<td class=described><a href="#UIstates">The UI element states
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E:checked<!--<br>E:indeterminate-->
<td class=meaning>a user interface element E which is
checked<!-- or in an
indeterminate state--> (for instance a
radio-button or checkbox)
<td class=described><a href="#UIstates">The UI element states
pseudo-classes</a>
<td class=origin>3
<tr>
<td class=pattern>E.warning
<td class=meaning>an E element whose class is "warning" (the document
language specifies how class is determined).
<td class=described><a href="#class-html">Class selectors</a>
<td class=origin>1
<tr>
<td class=pattern>E#myid
<td class=meaning>an E element with ID equal to "myid".
<td class=described><a href="#id-selectors">ID selectors</a>
<td class=origin>1
<tr>
<td class=pattern>E:not(s)
<td class=meaning>an E element that does not match simple selector s
<td class=described><a href="#negation">Negation pseudo-class</a>
<td class=origin>3
<tr>
<td class=pattern>E F
<td class=meaning>an F element descendant of an E element
<td class=described><a href="#descendant-combinators">Descendant
combinator</a>
<td class=origin>1
<tr>
<td class=pattern>E > F
<td class=meaning>an F element child of an E element
<td class=described><a href="#child-combinators">Child combinator</a>
<td class=origin>2
<tr>
<td class=pattern>E + F
<td class=meaning>an F element immediately preceded by an E element
<td class=described><a href="#adjacent-sibling-combinators">Adjacent
sibling combinator</a>
<td class=origin>2
<tr>
<td class=pattern>E ~ F
<td class=meaning>an F element preceded by an E element
<td class=described><a href="#general-sibling-combinators">General
sibling combinator</a>
<td class=origin>3
</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.
<h2 id=syntax><span class=secno>3. </span> Selector Syntax</h2>
<h3 id=structure><span class=secno>3.1. </span> Structure and Terminology</h3>
<p>A <dfn id=selector>selector</dfn> is a chain of one or more <a
href="#compound">compound selectors</a> separated by <a
href="#combinators">combinators</a>.
<p>A <dfn id=compound-selector><a name=compound>compound selector</a></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 selector</a>. No other type
selector or universal selector is allowed in the sequence.
<p>A <dfn id=simple-selector><a name=simple></a><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><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>An empty selector, containing no compound selector, is an <a
href="#invalid">invalid selector</a>.
<h3 id=subject><span class=secno>3.2. </span> Subject of a Selector</h3>
<p>The elements of a document tree that are represented by a selector are
the <dfn id=subjects>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>The subject of the selector can be explicitly identified by prepending
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 structures represented by these two selectors are the same, but the
subjects of the selectors are not.
</div>
<h3 id=case-sensitive><span class=secno>3.3. </span> 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 <a href="#CSS3NAMESPACE"
rel=biblioentry>[CSS3NAMESPACE]<!--{{!CSS3NAMESPACE}}--></a>.
<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. <a href="#CSS21"
rel=biblioentry>[CSS21]<!--{{!CSS21}}--></a>.
<h3 id=namespaces><span class=secno>3.4. </span> Namespaces</h3>
<p>Certain selectors support namespace prefixes. The mechanism by which
namespace prefixes are <dfn id=declared>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. <a href="#CSS3NAMESPACE"
rel=biblioentry>[CSS3NAMESPACE]<!--{{!CSS3NAMESPACE}}--></a>
<h3 id=invalid><span class=secno>3.5. </span> 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>a selector containing an invalid simple selector, an invalid
combinator or an invalid token is invalid.
<li>a group of selectors containing an invalid selector is invalid.
</ul>
<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><span class=secno>4. </span> Logical
Combinations</h2>
<h3 id=grouping><span class=secno>4.1. </span> Selector Groups</h3>
<p>A comma-separated list of selectors represents the union of all elements
selected by each of the individual selectors in the list. (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>
<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.
<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><span class=secno>4.2. </span> The Matches-Any Pseudo-class:
‘<code class=css>:matches()</code>’</h3>
<p>The matches pseudo-class, <code>:matches(<var>X</var>)</code>, is a
functional notation taking a <a href="#grouping">selector group</a> as an
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 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.</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>