Overview.bs

<pre class='metadata'>
Title: Selectors Level 4
Group: CSSWG
Shortname: selectors
Level: 4
Status: ED
Work Status: Revising
ED: https://drafts.csswg.org/selectors
TR: https://www.w3.org/TR/selectors4/
Previous Version: https://www.w3.org/TR/2013/WD-selectors4-20130502/
Previous Version: https://www.w3.org/TR/2012/WD-selectors4-20120823/
Previous Version: https://www.w3.org/TR/2011/WD-selectors4-20110929/
Editor: Elika J. Etemad / fantasai, Invited Expert, http://fantasai.inkedblade.net/contact, w3cid 35400
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
Former Editor: Tantek Çelik, http://www.tantek.com
Former Editor: Daniel Glazman
Former Editor: Ian Hickson
Former Editor: Peter Linss
Former Editor: John Williams
Abstract: <b>Selectors</b> 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 a 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.
Abstract: 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.
At Risk: the column combinator
At Risk: the '':drop()'' pseudo-class
At Risk: the '':read-write'' pseudo-class
Ignored Terms: function token, Document, DocumentFragment, math, h1, shadow tree, query(), quirks mode, button, a, span, object, p, div, q, area, link, label, input, html, em, li, ol, pre, CSS Value Definition Syntax
Ignored Vars: identifier, extended filtering
</pre>
<pre class=link-defaults>
spec:css-syntax-3; type:dfn; text:identifier
</pre>

<style>
#selector-examples td:first-child {
	white-space: nowrap;
}
</style>

<h2 id="context">
Introduction</h2>

	<em>This section is not normative.</em>

	A <dfn>selector</dfn> is a boolean predicate
	that takes an element in a tree structure
	and tests whether the element matches the selector or not.

	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 [[DOM]]

		<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.queryAll()</code> function defined in [[DOM]]
			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="https://en.wikipedia.org/wiki/Emmet_(software)">Emmet</a>.
	</ul>

	Selectors Levels 1, 2, and 3 are defined as the subsets of selector
	functionality defined in the <a href="https://www.w3.org/TR/REC-CSS1">CSS1</a>,
	<a href="https://www.w3.org/TR/CSS21/">CSS2.1</a>, and
	<a href="https://www.w3.org/TR/css3-selectors/">Selectors Level 3</a>
	specifications, respectively. This module defines Selectors Level 4.

<h3 id="placement">Module Interactions</h3>

	This module replaces the definitions of
	and extends the set of selectors defined for CSS in [[SELECT]] and [[CSS21]].

	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>

	<em>This section is non-normative, as it merely summarizes the
	following sections.</em>

	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.

	Selectors may range from simple element names to rich contextual
	representations.

	The following table summarizes the Selector syntax:

	<table class="data" id="selector-examples">
		<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>[[#the-universal-selector]]
				<td>2
			<tr>
				<td><code>E</code>
				<td>an element of type E
				<td>[[#type-selectors]]
				<td>1
		<tbody>
			<tr>
				<td><code>E:not(<var>s1</var>, <var>s2</var>)</code>
				<td>an E element that does not match either <a>compound selector</a> <var>s1</var>
						or <a>compound selector</a> <var>s2</var>
				<td>[[#negation]]
				<td>3/4
			<tr>
				<td><code>E:matches(<var>s1</var>, <var>s2</var>)</code>
				<td>an E element that matches <a>compound selector</a> <var>s1</var>
						and/or <a>compound selector</a> <var>s2</var>
				<td>[[#matches]]
				<td>4
			<tr>
				<td><code>E:has(<var>rs1</var>, <var>rs2</var>)</code>
				<td>an E element,
					if either of the <a>relative selectors</a> <var>rs1</var> or <var>rs2</var>,
					when evaluated with E as the <a>:scope elements</a>,
					match an element
				<td>[[#relational]]
				<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>[[#class-html]]
				<td>1
			<tr>
				<td><code>E#myid</code>
				<td>an E element with ID equal to <code>myid</code>.
				<td>[[#id-selectors]]
				<td>1
			<tr>
				<td><code>E[foo]</code>
				<td>an E element with a <code>foo</code> attribute
				<td>[[#attribute-selectors]]
				<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>[[#attribute-selectors]]
				<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>[[#attribute-case]]
				<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>[[#attribute-selectors]]
				<td>2
			<tr>
				<td><code>E[foo^="bar"]</code>
				<td>an E element whose <code>foo</code> attribute value
						begins exactly with the string <code>bar</code>
				<td>[[#attribute-substrings]]
				<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>[[#attribute-substrings]]
				<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>[[#attribute-substrings]]
				<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>[[#attribute-selectors]]
				<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>[[#the-dir-pseudo]]
				<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 otherwise written with traditional Chinese characters
				<td>[[#the-lang-pseudo]]
				<td>2/4
		<tbody>
			<tr>
				<td><code>E:any-link</code>
				<td>an E element being the source anchor of a hyperlink
				<td>[[#the-any-link-pseudo]]
				<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>[[#link]]
				<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>[[#link]]
				<td>1
			<tr>
				<td><code>E:target</code>
				<td>an E element being the target of the referring URL
				<td>[[#the-target-pseudo]]
				<td>3
			<tr>
				<td><code>E:scope</code>
				<td>an E element being a designated reference element
				<td>[[#the-scope-pseudo]]
				<td>4
		<tbody>
			<tr>
				<td><code>E:current</code>
				<td>an E element that is currently presented in a time-dimensional canvas
				<td>[[#time-pseudos]]
				<td>4
			<tr>
				<td><code>E:current(<var>s</var>)</code>
				<td>an E element that is the deepest '':current'' element that
						matches selector <var>s</var>
				<td>[[#time-pseudos]]
				<td>4
			<tr>
				<td><code>E:past</code>
				<td>an E element that is in the past in a time-dimensional canvas
				<td>[[#time-pseudos]]
				<td>4
			<tr>
				<td><code>E:future</code>
				<td>an E element that is in the future in a time-dimensional canvas
				<td>[[#time-pseudos]]
				<td>4
		<tbody>
			<tr>
				<td><code>E:active</code>
				<td>an E element that is in an activated state
				<td>[[#useraction-pseudos]]
				<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>[[#useraction-pseudos]]
				<td>2
			<tr>
				<td><code>E:focus</code>
				<td>an E element that has user input focus
				<td>[[#useraction-pseudos]]
				<td>2
			<tr>
				<td><code>E:focus-ring</code>
				<td>an E element that has user input focus,
					and the UA has determined that a focus ring or other indicator
					should be drawn for that element
				<td>[[#useraction-pseudos]]
				<td>4
			<tr>
				<td><code>E:drop</code>
				<td>an E element that can possibly receive a drop
				<td>[[#drag-pseudos]]
				<td>4
			<tr>
				<td><code>E:drop(active)</code>
				<td>an E element that is the current drop target for the item being dragged
				<td>[[#drag-pseudos]]
				<td>4
			<tr>
				<td><code>E:drop(valid)</code>
				<td>an E element that could receive the item currently being dragged
				<td>[[#drag-pseudos]]
				<td>4
			<tr>
				<td><code>E:drop(invalid)</code>
				<td>an E element that cannot receive the item currently being dragged, but could receive some other item
				<td>[[#drag-pseudos]]
				<td>4
		<tbody>
			<tr>
				<td><code>E:enabled<br>E:disabled</code>
				<td>a user interface element E that is enabled or disabled, respectively
				<td>[[#enableddisabled]]
				<td>3
			<tr>
				<td><code>E:read-write</code><br><code>E:read-only</code>
				<td>a user interface element E that is user alterable, or not
				<td>[[#rw-pseudos]]
				<td>3-UI/4
			<tr>
				<td><code>E:placeholder-shown</code>
				<td>an input control currently showing placeholder text
				<td>[[#rw-pseudos]]
				<td>3-UI/4
			<tr>
				<td><code>E:default</code>
				<td>a user interface element E that is the default item in a group of related choices
				<td>[[#the-default-pseudo]]
				<td>3-UI/4
			<tr>
				<td><code>E:checked</code>
				<td>a user interface element E that is checked/selected
					 (for instance a radio-button or checkbox)
				<td>[[#checked]]
				<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>[[#indeterminate]]
				<td>4
			<tr>
				<td><code>E:valid</code><br><code>E:invalid</code>
				<td>a user-input element E that meets, or doesn't, its data validity semantics
				<td>[[#range-pseudos]]
				<td>3-UI/4
			<tr>
				<td><code>E:in-range</code><br><code>E:out-of-range</code>
				<td>a user-input element E whose value is in-range/out-of-range
				<td>[[#range-pseudos]]
				<td>3-UI/4
			<tr>
				<td><code>E:required</code><br><code>E:optional</code>
				<td>a user-input element E that requires/does not require input
				<td>[[#opt-pseudos]]
				<td>3-UI/4
			<tr>
				<td><code>E:user-error</code>
				<td>a user-altered user-input element E with incorrect input (invalid, out-of-range, omitted-but-required)
				<td>[[#user-pseudos]]
				<td>4
		<tbody>
			<tr>
				<td><code>E:root</code>
				<td>an E element, root of the document
				<td>[[#structural-pseudos]]
				<td>3
			<tr>
				<td><code>E:empty</code>
				<td>an E element that has no children (not even text nodes)
				<td>[[#structural-pseudos]]
				<td>3
			<tr>
				<td><code>E:blank</code>
				<td>an E element that has no content except maybe white space
				<td>[[#structural-pseudos]]
				<td>4
			<tr>
				<td><code>E:nth-child(<var>n</var> [of <var>S</var>]?)</code>
				<td>an E element, the <var>n</var>-th child of its parent matching <var>S</var>
				<td>[[#child-index]]
				<td>3
			<tr>
				<td><code>E:nth-last-child(<var>n</var> [of <var>S</var>]?)</code>
				<td>an E element, the <var>n</var>-th child of its parent matching <var>S</var>,
						counting from the last one
				<td>[[#child-index]]
				<td>3
			<tr>
				<td><code>E:first-child</code>
				<td>an E element, first child of its parent
				<td>[[#child-index]]
				<td>2
			<tr>
				<td><code>E:last-child</code>
				<td>an E element, last child of its parent
				<td>[[#child-index]]
				<td>3
			<tr>
				<td><code>E:only-child</code>
				<td>an E element, only child of its parent
				<td>[[#child-index]]
				<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>[[#typed-child-index]]
				<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>[[#typed-child-index]]
				<td>3
			<tr>
				<td><code>E:first-of-type</code>
				<td>an E element, first sibling of its type
				<td>[[#typed-child-index]]
				<td>3
			<tr>
				<td><code>E:last-of-type</code>
				<td>an E element, last sibling of its type
				<td>[[#typed-child-index]]
				<td>3
			<tr>
				<td><code>E:only-of-type</code>
				<td>an E element, only sibling of its type
				<td>[[#typed-child-index]]
				<td>3
		<tbody>
			<tr>
				<td><code>E F</code> <em>or</em> <code>E >> F</code>
				<td>an F element descendant of an E element
				<td>[[#descendant-combinators]]
				<td>1 <em>or</em> 4
			<tr>
				<td><code>E > F</code>
				<td>an F element child of an E element
				<td>[[#child-combinators]]
				<td>2
			<tr>
				<td><code>E + F</code>
				<td>an F element immediately preceded by an E element
				<td>[[#adjacent-sibling-combinators]]
				<td>2
			<tr>
				<td><code>E ~ F</code>
				<td>an F element preceded by an E element
				<td>[[#general-sibling-combinators]]
				<td>3
		<tbody>
			<tr>
				<td><code>F || E</code>
				<td>an E element that represents a cell in a grid/table
						belonging to a column represented by an element F
				<td>[[#table-pseudos]]
				<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>[[#table-pseudos]]
				<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>[[#table-pseudos]]
				<td>4
	</table>

	Note: Some Level 4 selectors (noted above as "3-UI") were introduced in [[CSS3UI]].

<h3 id="profiles">
<a>Dynamic</a> vs <a>Static</a> Selector Profiles</h3>

	Selectors are used in many different contexts,
	with wildly varying performance characteristics.
	Some powerful selectors are unfortunately too slow
	to realistically include in the more performance-sensitive contexts.
	To accommodate this, two profiles of the Selectors spec are defined:

	<dl>
		<dt><dfn local-lt="dynamic">dynamic profile</dfn>
		<dd>
			The <a>dynamic</a> profile is appropriate for use in any context,
			including dynamic browser CSS selector matching.
			It includes every selector defined in this document,
			except for:

			<ul>
				<li>The '':has()'' pseudo-class
			</ul>

		<dt><dfn local-lt="static">static profile</dfn>
		<dd>
			The <a>static</a> profile is appropriate for contexts which aren't extremely performance sensitive;
			in particular, it's appropriate for contexts which evaluate selectors against a static document tree.
			For example, the {{Element/query()}} method defined in [[DOM]] should use the <a>static</a> profile.
			It includes all of the selectors defined in this document.
	</dl>

	CSS implementations conformant to Selectors Level 4 must use the <a>dynamic</a> profile for CSS selection.
	Implementations using the <a>dynamic</a> profile must treat selectors that are not included in the profile
	as unknown and invalid.

	<p class='issue'>
		The categorization of things into the "dynamic" or "static" profiles needs implementor review.
		If some things currently not in the dynamic profile can reasonably be done in CSS Selectors,
		we should move them.


<h2 id="syntax">
Selector Syntax and Structure</h2>

<h3 id="structure">
Structure and Terminology</h3>

	The term <a>selector</a> can refer to a <a>simple selector</a>,
	<a>compound selector</a>, <a>complex selector</a>, or <a>selector list</a>.

	A <dfn id="simple" export>simple selector</dfn>
	represents an element matched by a particular aspect.
	A <a>type selector</a>,
	<a>universal selector</a>,
	<a>attribute selector</a>,
	<a>class selector</a>,
	<a>ID selector</a>,
	or <a>pseudo-class</a>
	is a <a>simple selector</a>.

	A <dfn id="compound" export>compound selector</dfn>
	is a sequence of <a>simple selectors</a>
	that are not separated by a <a>combinator</a>.
	It represents an element that matches all of the <a>simple selectors</a> it contains.
	If it contains a <a>type selector</a> or <a>universal selector</a>,
	that selector must come first in the sequence.
	Only one type selector or universal selector is allowed in the sequence.

	Note: As whitespace is a valid <a>combinator</a>,
	no whitespace is allowed between the <a>simple selectors</a>
	in a <a>compound selector</a>.

	A <dfn id="complex" export>complex selector</dfn> is a sequence of one or more <a>compound selectors</a>
	separated by <a>combinators</a>.

	A <dfn export for=selector>combinator</dfn> represents a particular kind of relationship
	between the elements matched by the <a>compound selectors</a> on either side.
	Combinators in Selectors level 4 include:
	the <a>descendant combinator</a> (white space),
	the <a>child combinator</a> (U+003E, <code>></code>),
	the <a>next-sibling combinator</a> (U+002B, <code>+</code>),
	and the <a>subsequent-sibling combinator</a> (U+007E, <code>~</code>).

	A <dfn export lt="list of simple selectors|list of compound selectors|list of complex selectors">list of simple/compound/complex selectors</dfn>
	is a comma-separated list of
	<a lt="simple selector">simple</a>,
	<a lt="compound selector">compound</a>,
	or <a>complex selectors</a>.
	This is also called just a <dfn export lt="selector list|list of selectors">selector list</dfn>
	when the type is either unimportant or specified in the surrounding prose;
	if the type is important and unspecified,
	it defaults to meaning a <a>list of complex selectors</a>.
	See [[#grouping]] for further details on <a>selector lists</a>.

	The <dfn export for=selector lt="subject|subject of a selector">subject of a selector</dfn> is
	the element(s) that selector is defined to be about:

	<ul>
		<li>For a <a>simple selector</a>, it is any element represented by that selector.
		<li>For a <a>compound selector</a>, it is any element that matches all of its composite <a>simple selectors</a>.
		<li>For a <a>complex selector</a>, it is any element that matches the last <a>compound selector</a> in the selector.
		<li>For a <a>selector list</a>, it is any element that matches any of the <a>selectors</a> in the list.
	</ul>

	An element is said to <dfn for=selector export>match</dfn> a selector
	if it is a <a>subject</a> of that selector.
	Thus a selector consisting of a single <a>compound selector</a>
	matches any element satisfying its requirements.
	Prepending another <a>compound selector</a> and a <a>combinator</a>
	to a sequence imposes additional matching constraints,
	so the <a>subjects</a> of a <a>complex selector</a> are always a subset of
	the elements represented by its last <a>compound selector</a>.

	Issue: Can we make this less wishy-washy by saying that a complex selector “represents” its last compound selector?
	Or do we need to have ''a + b'' “represent” both an "a" and a "b" in a sibling relationship?

	Issue: Pseudo-elements aren't handled here, and should be.


<h3 id='data-model'>
Data Model</h3>

	Selectors are evaluated against an element tree such as the DOM. [[!DOM]]
	Within this specification,
	this may be referred to as the "document tree" or "source document".

	Each element may have any of the following five aspects,
	which can be selected against,
	all of which are matched as strings:

	<ul>
		<li>The element's type (also known as its tag name).
		<li>The element's namespace.
		<li>An ID.
		<li>Classes (named groups) to which it belongs.
		<li>Attributes, which are name-value pairs.
	</ul>

	While individual elements may lack any of the above features,
	some elements are <dfn export>featureless</dfn>.
	A <a>featureless</a> element does not match any selector at all,
	except those it is explicitly defined to match.
	If a given selector <em>is</em> allowed to match a <a>featureless</a> element,
	it must do so while ignoring the default namespace. [[CSS3NAMESPACE]]

	<div class='example'>
		For example, the <a>shadow host</a> in a <a>shadow tree</a> is <a>featureless</a>,
		and can't be matched by <em>any</em> <a>pseudo-class</a> except for '':host'' and '':host-context()''.)
	</div>

	Many of the selectors depend on the semantics of the <dfn>document language</dfn>
	(i.e. the language and semantics of the document tree)
	and/or the semantics of the <dfn>host language</dfn>
	(i.e. the language that is using selectors syntax).
	For example, the '':lang()'' selector depends on the <a>document language</a> (e.g. HTML)
	to define how an element is associated with a language.
	As a slightly different example, the ''::first-line'' pseudo-element
	depends on the <a>host language</a> (e.g. CSS)
	to define what a ''::first-line'' pseudo-element represents
	and what it can do.

<h3 id="scoping">
Scoped Selectors</h3>

	Some host applications may choose to <dfn export lt="scoped selector" local-lt="scope">scope</dfn> selectors
	to a particular subtree or fragment of the document.
	The root of the scoping subtree is called the <dfn export>scoping root</dfn>,
	and may be either a true element (the <dfn export>scoping element</dfn>)
	or a <dfn export lt="virtual scoping root">virtual</dfn> one (such as a <a interface>DocumentFragment</a>).

	When a selector is <a>scoped</a>,
	it matches an element only if the element is a descendant of the <a>scoping root</a>.
	(The rest of the selector can match unrestricted;
	it's only the final matched elements that must be within the scope.)

	<div class='example'>
		For example,
		the <code>element.querySelector()</code> function defined in [[DOM]]
		allows the author to evaluate a <a>scoped</a> selector
		relative to the <code>element</code> it's called on.

		A call like <code highlight=js>widget.querySelector("a")</code>
		will thus only find <{a}> elements inside of the <code>widget</code> element,
		ignoring any other <{a}>s that might be scattered throughout the document.
	</div>

	Note: If the context does not explicitly define any <a>:scope elements</a> for the selector,
	the <a>scoping root</a> is a <a>:scope element</a>.

<h3 id="relative">
Relative Selectors</h3>

	Certain contexts may accept <dfn lt="relative selector | relative | scope-relative" export>relative selectors</dfn>,
	which are a shorthand for selectors that represent elements relative to a <a>:scope element</a>
	(i.e. an element that matches '':scope'').
	In a <a>relative selector</a>,
	“:scope ” (the '':scope'' pseudo-class followed by a space)
	is implied at the beginning of each <a>complex selector</a>
	that does not already contain the '':scope'' pseudo-class.
	This allows the selector to begin syntactically with a <a>combinator</a>.
	However, it must be <a href="#absolutize">absolutized</a> before matching.

	Relative selectors, once absolutized,
	can additionally be <a>scoped</a>.

<h4 id='absolutizing'>
Absolutizing a Relative Selector</h4>

	To <dfn id='absolutize' export>absolutize a relative selector</dfn>:

	If there are no <a>:scope elements</a>
	and the selector is <a>scoped</a> to a <a>virtual scoping root</a>:

	<ol>
		<li>
			If the selector starts with a <a>child combinator</a>,
			remove the child combinator.
			The selector is now absolute,
			with the additional constraint that the first compound selector in the selector
			only matches elements without a parent.

		<li>
			Otherwise, if the selector starts with any combinator other than the white space form of the <a>descendant combinator</a>,
			change the selector to '':not(*)''.
			<span class='note'>This is the shortest selector that is valid, but guaranteed to match nothing.</span>

		<li>
			Otherwise, the selector is already absolute.
	</ol>

	Otherwise:

	<ol>
		<li>
			If the selector starts with a <a>combinator</a> other than the white space form of the <a>descendant combinator</a>,
			prepend '':scope'' as the initial <a>compound selector</a>.

		<li>
			Otherwise, if the selector does not contain any instance of the '':scope'' pseudo-class
			(either at the top-level or as an argument to a functional pseudo-class),
			prepend '':scope'' followed by the white space form of the <a>descendant combinator</a>.

		<li>
			Otherwise, the selector is already absolute.
	</ol>

	To <dfn id='absolutize-list' export>absolutize a relative selector list</dfn>,
	absolutize each relative selector in the list.


<h3 id="pseudo-classes">
Pseudo-classes</h3>

	<dfn export id="pseudo-class" lt="pseudo-class">Pseudo-classes</dfn> are <a>simple selectors</a>
	that permit selection based on
	information that lies outside of the document tree
	or that can be awkward or impossible to express using the other simple selectors.
	They can also be dynamic,
	in the sense that an element can acquire or lose a pseudo-class
	while a user interacts with the document,
	without the document itself changing.
	<a>Pseudo-classes</a> do not appear in or modify the document source or document tree.

	The syntax of a <a>pseudo-class</a>
	consists of a ":" (U+003A COLON)
	followed by the name of the <a>pseudo-class</a>
	as a CSS <a>identifier</a>,
	and, in the case of a <dfn export id="functional-pseudo-class">functional pseudo-class</dfn>,
	a pair of parentheses containing its arguments.

	<p class="example">
		For example, '':valid'' is a regular pseudo-class,
		and  '':lang()'' is a <a>functional pseudo-class</a>.

	Like all CSS keywords, <a>pseudo-class</a> names are <a>ASCII case-insensitive</a>.
	No <a>white space</a> is allowed between the colon and the name of the <a>pseudo-class</a>,
	nor, as usual for CSS syntax,
	between a <a>functional pseudo-class</a>’s name and its opening parenthesis
	(which thus form a CSS <a>function token</a>).
	Also as usual,
	<a>white space</a> is allowed around the arguments inside the parentheses
	of a functional pseudo-class
	unless otherwise specified.

	Like other <a>simple selectors</a>,
	<a>pseudo-classes</a> are allowed in all <a>compound selectors</a> contained in a selector,
	and must follow the <a>type selector</a> or <a>universal selector</a>, if present.

	Note: Some <a>pseudo-classes</a> are mutually exclusive
	(such that a <a>compound selector</a> containing them, while valid, will never match anything),
	while others can apply simultaneously to the same element.

<h3 id="pseudo-elements">Pseudo-elements</h3>

	Similar to how certain <a>pseudo-classes</a> represent additional state information
	not directly present in the document tree,
	a <dfn export id="pseudo-element">pseudo-element</dfn> represents an <em>element</em>
	not directly present in the document tree.
	They are used to create abstractions about the document tree
	beyond those provided by the document tree.
	For example,
	pseudo-elements can be used to select portions of the document
	that do not correspond to a document-language element
	(including such ranges as don't align to element boundaries or fit within its tree structure);
	that represent content not in the document tree or in an alternate projection of the document tree;
	or that rely on information provided by styling, layout, user interaction, and other processes that are not reflected in the document tree.

	<div class="example">
		For instance, document languages do not offer mechanisms to access
		the first letter or first line of an element's content,
		but there exist <a>pseudo-elements</a>
		(''::first-letter'' and ''::first-line'')
		that allow those things to be styled.
		Notice especially that in the case of ''::first-line'',
		which portion of content is represented by the pseudo-element
		depends on layout information
		that cannot be inferred from the document tree.

		<a>Pseudo-elements</a> can also represent content that doesn't exist in the source document at all,
		such as the ''::before'' and ''::after'' pseudo-elements
		which allow additional content to be inserted before or after the contents of any element.
	</div>

	Like <a>pseudo-classes</a>
	<a>pseudo-elements</a> do not appear in or modify the document source or document tree.
	Accordingly, they also do not affect the interpretation of <a>structural pseudo-classes</a>
	or other selectors pertaining to their <a>originating element</a> or its tree.

	The host language defines which pseudo-elements exist, their type, and their abilities.
	Pseudo-elements that exist in CSS
	are defined in [[CSS21]] (Level 2), [[SELECT]] (Level 3), and [[CSS-PSEUDO-4]] (Level 4).

<h4 id="pseudo-element syntax">
Syntax</h4>

	The syntax of a <a>pseudo-element</a>
	is "::" (two U+003A COLON characters)
	followed by the name of the <a>pseudo-element</a> as an <a>identifier</a>.
	<a>Pseudo-element</a> names are <a>ASCII case-insensitive</a>.
	No <a>white space</a> is allowed between the two colons, or between the colons and the name.

	Because <a href="https://www.w3.org/TR/CSS1">CSS Level 1</a> and <a href="https://www.w3.org/TR/CSS2">CSS Level 2</a>
	conflated pseudo-elements and pseudo-classes by sharing a single-colon syntax for both,
	user agents must also accept the previous one-colon notation
	for the Level 1 &amp; 2 pseudo-elements
	(''::before'', ''::after'', ''::first-line'', and ''::first-letter'').
	This compatibility notation is not allowed any other <a>pseudo-elements</a>.
	However, as this syntax is deprecated,
	authors should use the Level 3+ double-colon syntax for these <a>pseudo-elements</a>.

	<a>Pseudo-elements</a> are <a>featureless</a>,
	and so can't be matched by any other selector.

<h4 id="pseudo-element-attachment">
Binding to the Document Tree</h4>

	<a>Pseudo-elements</a> do not exist independently in the tree:
	they are always bound to another element on the page,
	called their <dfn export>originating element</dfn>.
	Syntactically, a <a>pseudo-element</a> immediately follows
	the <a>compound selector</a> representing its <a>originating element</a>.
	If this <a>compound selector</a> is omitted,
	it is assumed to be the <a>universal selector</a> ''*''.

	<div class='example'>
		For example, in the selector ''div a::before'',
		the ''a'' elements matched by the selector are the <a>originating elements</a>
		for the ''::before'' pseudo-elements attached to them.

		The selector ''::first-line'' is equivalent to ''*::first-line'',
		which selects the ''::first-line'' pseudo-element on <em>every</em> element in the document.
	</div>

	When a <a>pseudo-element</a> is encountered in a selector,
	the part of the selector before the <a>pseudo-element</a> selects the <a>originating element</a> for the <a>pseudo-element</a>;
	the part of the selector after it, if any, applies to the <a>pseudo-element</a> itself.
	(See below.)

<h4 id="pseudo-element-states">
Pseudo-classing Pseudo-elements</h4>

	A <a>pseudo-element</a> may be immediately followed
	by any combination of the <a href="#useraction-pseudos">user action pseudo-classes</a>,
	in which case the <a>pseudo-element</a> is represented only when it is in the corresponding state.
	Whether these pseudo-classes can match on the <a>pseudo-element</a>
	depends on the <a>pseudo-class</a> and <a>pseudo-element</a>&rdquo;s definitions:
	unless otherwise-specified, none of these <a>pseudo-classes</a>
	will match on the <a>pseudo-element</a>.

	Issue: Clarify that '':not()'' and '':matches()'' can be used when containing above-mentioned pseudos.

	<div class="example">
		For example, since the '':hover'' pseudo-class specifies
		that it can apply to any pseudo-element,
		''::first-line:hover'' will match when the first line is hovered.
		However, since neither '':focus'' nor ''::first-line''
		define that '':focus'' can apply to ''::first-line'',
		the selector ''::first-line:focus'' will never match anything.

		Issue: Does ''::first-line:not(:focus)'' match anything?

		Notice that ''::first-line:hover'' is is very different from '':hover::first-line'',
		which matches the first line of any originating element that is hovered!
		For example, '':hover::first-line'' also matches the first line of a paragraph
		when the second line of the paragraph is hovered,
		whereas ''::first-line:hover'' only matches if the first line itself is hovered.
	</div>

	Note: Note that, unless otherwise specified in a future specification,
	pseudo-classes other than the <a href="#useraction-pseudos">user action pseudo-classes</a>
	are not valid when compounded to a pseudo-element;
	so, for example, ''::before:first-child'' is an invalid selector.

<h4 id="pseudo-element-structure">
Internal Structure</h4>

	Some <a>pseudo-elements</a> are defined to have internal structure.
	These <a>pseudo-elements</a> may be followed by child/descendant combinators
	to express those relationships.
	Selectors containing <a>combinators</a> after the pseudo-element
	are otherwise invalid.

	<div class="example">
		For example, ''::first-letter + span'' and ''::first-letter em'' are invalid selectors.
		However, since ''::shadow'' is defined to have internal structure,
		''::shadow > p'' is a valid selector.
	</div>

	Note: A future specification may expand the capabilities of existing pseudo-elements,
	so some of these currently-invalid selectors (e.g. ''::first-line :any-link'')
	may become valid in the future.

	The children of such <a>pseudo-elements</a> can simultaneously be children of other elements, too.
	However, at least in CSS, their rendering must be defined so as to maintain the tree-ness of the <a>box tree</a>.

	<div class='example'>
		For example,
		the ''::content'' pseudo-element treats elements distributed to it as its children.
		This means that, given the following fragment:

		<pre>
		&lt;div>
			&lt;span>foo&lt;/span>
			&lt;"shadow root">
				&lt;content>&lt;/content>
			&lt;/"shadow root">
		&lt;/div>
		</pre>

		the selectors ''div > span'' and ''div::shadow ::content > span'' select the same element via different paths.

		However, when rendered,
		the <code>&lt;span></code> element generates boxes as if it were the child of the <code>&lt;content></code> element,
		rather than the <code>&lt;div></code> element,
		so the tree structure of the <a>box tree</a> is maintained.
	</div>

<h3 id="case-sensitive">
Characters and case sensitivity</h3>

	All Selectors syntax is case-insensitive within the ASCII range
	(i.e. [a-z]  and \[A-Z] are equivalent),
	except for the following parts,
	which are not under the control of Selectors:
	the case-sensitivity of
	document language element names,
	attribute names,
	and attribute values
	depends on the document language.

	<div class=example>
		For example,
		<a href="http://www.whatwg.org/html/selectors.html#case-sensitivity">in HTML, element and attribute names are ASCII case-insensitive</a>,
		but in XML, they are case-sensitive.
	</div>

	Case sensitivity of namespace prefixes is defined in [[!CSS3NAMESPACE]].
	Case sensitivity of <a>language ranges</a> is defined in the '':lang()'' section.

	<dfn id="whitespace">White space</dfn> in Selectors consists of the
	code points 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 code points, such as EM SPACE (U+2003) and
	IDEOGRAPHIC SPACE (U+3000), are never part of white space.

	Code points in Selectors can be escaped with a backslash
	according to the same <a href="https://www.w3.org/TR/CSS21/syndata.html#characters">escaping rules</a> as CSS. [[!CSS21]]
	Note that escaping a code point "cancels out" any special meaning it may have in Selectors.
	For example, the selector ''#foo>a'' contains a combinator,
	but ''#foo\>a'' instead selects an element with the id <code>foo>a</code>.


<h3 id="namespaces">
Declaring Namespace Prefixes</h3>

	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 ''@namespace''rule. [[!CSS3NAMESPACE]]

<h3 id="invalid">
Invalid Selectors and Error Handling</h3>

	User agents must observe the rules for handling
	<dfn export lt="invalid selector" local-lt="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.
		<li>an empty selector, i.e. one that contains no <a>compound selector</a>, is invalid.
	</ul>

	An <a>invalid selector</a> represents, and therefore matches, nothing.

<h2 id="logical-combination">
Logical Combinations</h2>

<h3 id="grouping">
Selector Lists</h3>

	A comma-separated list of selectors represents the union of all
	elements selected by each of the individual selectors in the
	<a>selector list</a>.
	(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">
		CSS example:
		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>
		is equivalent to:
		<pre>h1, h2, h3 { font-family: sans-serif }
		</pre>
	</div>

	<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 <a>selector list</a> 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">
		Invalid CSS example:

		<pre>
		h1 { font-family: sans-serif }
		h2..foo { font-family: sans-serif }
		h3 { font-family: sans-serif }
		</pre>

		is not equivalent to:

		<pre>h1, h2..foo, h3 { font-family: sans-serif } </pre>

		because the above selector (''h1, h2..foo, h3'')
		is entirely invalid and the entire style rule is dropped. (When
		the selectors are not grouped, only the rule for ''h2..foo''
		is dropped.)
	</div>

<h3 id="matches">
The Matches-any Pseudo-class: '':matches()''</h3>

	The matches-any pseudo-class, <dfn id='matches-pseudo'>:matches()</dfn>,
	is a functional pseudo-class taking a <a>selector list</a>
	as its argument. It represents an element that is represented by its argument.

	Pseudo-elements cannot be represented by the matches-any pseudo-class;
	they are not valid within '':matches()''.

	Default namespace declarations do not affect the <a>compound selector</a>
	representing the <a>subject</a> of any selector
	within a '':matches()'' pseudo-class,
	unless that compound selector contains
	an explicit <a>universal selector</a> or <a>type selector</a>.

	<div class="example">
		For example, the 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>

		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 '':matches()'' notation:

		<pre>*|*:matches(*:hover, *:focus) </pre>
	</div>

<h3 id="is">
The Specificity-adjustment Pseudo-class: '':is()''</h3>

The Specificity-adjustment pseudo-class '':is()'', is a functional pseudo-class
with the same syntax and functionality as '':matches()''.
Unlike '':matches()'', neither the pseudo-class, nor any of its arguments
contribute to the specificity of the selector.

This is useful for introducing filters in a selector while keeping it easy to override.

<div class="example">
	Below is a common example where the specificity heuristic fails
	to match author expectations:

	<pre>
		a:not(:hover) {
			text-decoration: none;
		}

		nav a {
			/* Has no effect */
			text-decoration: underline;
		}
	</pre>

	However, by using '':is()'' the author can explicitly declare their intent:

	<pre>
		a:is(:not(:hover)) {
			text-decoration: none;
		}

		nav a {
			/* Works now! */
			text-decoration: underline;
		}
	</pre>
</div>

Note: Future versions of this specification may introduce a second argument to explicitly set
the specificity of each instance of this pseudo-class, which will default to zero.

<h3 id="negation">
The Negation Pseudo-class: '':not()''</h3>

	The negation pseudo-class, <dfn id='negation-pseudo'>:not()</dfn>,
	is a functional pseudo-class taking a <a>selector list</a> as an argument.
	It represents an element that is not represented by its argument.

	Note: In Selectors Level 3,
	only a single <a>simple selector</a> was allowed as the argument to '':not()''.

	Pseudo-elements cannot be represented by the negation pseudo-class;
	they are not valid within '':not()''.

	<div class="example">
		For example, the following selector matches all <a element>button</a> elements in an HTML document
		that are not disabled.

		<pre>button:not(\[DISABLED]) </pre>

		The following selector represents all but FOO elements.

		<pre>*:not(FOO)</pre>

		The following compound selector represents all HTML elements
		except links.

		<pre>html|*:not(:link):not(:visited)</pre>
	</div>

	As with '':matches()'',
	default namespace declarations do not affect the <a>compound selector</a>
	representing the <a>subject</a> of any selector
	within a '':not()'' pseudo-class,
	unless that compound selector contains
	an explicit <a>universal selector</a> or <a>type selector</a>.
	(See '':matches()'' for examples.)

	Note: The '':not()'' pseudo-class allows useless selectors to be written.
	For instance '':not(*|*)'', which represents no element at all,
	or ''div:not(span)'', which is equivalent to ''div'' but with a higher specificity.

<h3 id='relational'>
The Relational Pseudo-class: '':has()''</h3>

	The relational pseudo-class, <dfn id='has-pseudo'>:has()</dfn>,
	is a functional pseudo-class taking a <a>relative selector</a> <a lt="selector list">list</a> as an argument.
	It represents an element if any of the <a>relative selectors</a>,
	when <a href="#absolutize">absolutized</a>
	and evaluated with the element as the <a>:scope elements</a>,
	would match at least one element.

	<div class='example'>
		For example, the following selector matches only
		<code>&lt;a></code> elements that contain an <code>&lt;img></code> child:

		<pre>a:has(> img)</pre>

		The following selector matches a <code>&lt;dt></code> element
		immediately followed by another <code>&lt;dt></code> element:

		<pre>dt:has(+ dt)</pre>

		The following selector matches <code>&lt;section></code> elements
		that don't contain any heading elements:

		<pre>section:not(:has(h1, h2, h3, h4, h5, h6))</pre>

		Note that ordering matters in the above selector.
		Swapping the nesting of the two pseudo-classes, like:

		<pre>section:has(:not(h1, h2, h3, h4, h5, h6))</pre>

		...would result matching any <code>&lt;section></code> element
		which contains anything that's not a header element.
	</div>


<h2 id="elemental-selectors">
Elemental selectors</h2>

<h3 id="type-selectors">
Type (tag name) selector</h3>

	A <dfn export>type selector</dfn> is the name of a document language element type,
	and represents an instance of that element type in the document tree.

	<div class="example">
		For example, the selector ''h1'' represents an <a element>h1</a> element in the document.
	</div>

	A <a>type selector</a> is written as a <a>CSS qualified name</a>:
	an <a>identifier</a> with an optional namespace prefix.
	[[!CSS3NAMESPACE]]
	(See [[#type-nmsp]].)

<h3 id="the-universal-selector">
Universal selector </h3>

	The <dfn export>universal selector</dfn> is a special <a>type selector</a>,
	that represents an element of any element type.

	It is written a <a>CSS qualified name</a>
	with an asterisk (<code>*</code> U+002A) as the local name.
	Like a <a>type selector</a>,
	the <a>universal selector</a> can be qualified by a namespace,
	restricting it to only elements belonging to that namespace,
	and is affected by a default namespace as defined in [[#type-nmsp]].

	Unless an element is <a>featureless</a>,
	the presence of a <a>universal selector</a> has no effect on whether the element matches the selector.
	(<a>Featureless</a> elements do not match any selector,
		including the <a>universal selector</a>.)

	<div class="example">
		<ul>
			<li>
				<css>*[hreflang|=en]</css> and <css>[hreflang|=en]</css> are equivalent,
			<li>''*.warning'' and ''.warning'' are equivalent,
			<li>''*#myid'' and ''#myid'' are equivalent.
		</ul>
	</div>

	The <a>universal selector</a> follows the same syntax rules as other <a>type selectors</a>:
	only one can appear per <a>compound selector</a>,
	and it must be the first <a>simple selector</a> in the <a>compound selector</a>.

	Note: In some cases, adding a <a>universal selector</a> can make a selector easier to read,
	even though it has no effect on the matching behavior.
	For example, ''div :first-child'' and ''div:first-child'' are somewhat difficult to tell apart at a quick glance,
	but writing the former as ''div *:first-child'' makes the difference obvious.

<h3 id='type-nmsp'>
Namespaces in Elemental Selectors</h3>

	<a>Type selectors</a> and <a>universal selectors</a> 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” (<code>|</code> U+007C).
	(See, e.g., [[XML-NAMES]] for the use of namespaces in XML.)
	It has the following meaning in each form:

	<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">
		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>

		The first rule (not counting the ''@namespace'' at-rule)
		will match only <a element>h1</a> elements in the
		"http://www.example.com" namespace.

		The second rule will match all elements in the
		"http://www.example.com" namespace.

		The third rule will match only <a element>h1</a> elements with
		no namespace.

		The fourth rule will match <a element>h1</a> elements in any
		namespace (including those without any namespace).

		The last rule is equivalent to the fourth rule because no default
		namespace has been defined.
	</div>

	If a <a>default namespace</a> is declared,
	<a>compound selectors</a> without <a>type selectors</a> in them
	still only match elements in that default namespace.

	<div class='example'>
		For example,
		in the following style sheet:

		<pre>
		@namespace url("http://example.com/foo");

		.special { ... }
		</pre>

		The ''.special'' selector only matches elements in the "http://example.com/foo" namespace,
		even though no reference to the type name (which is paired with the namespace in the DOM) appeared.
	</div>

	A <a>type selector</a> or <a>universal selector</a> containing a namespace prefix
	that has not been previously <a href="#nsdecl">declared</a>
	is an <a>invalid selector</a>.

<h2 id="attribute-selectors">
Attribute selectors</h2>

	Selectors allow the representation of an element's attributes. When
	a selector is used as an expression to match against an element,
	an <dfn export>attribute selector</dfn> must be considered to match an element if that
	element has an attribute that matches the attribute represented by the
	attribute selector.

	<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">
Attribute presence and value selectors</h3>

	CSS2 introduced four attribute selectors:

	<dl>
		<dt>''[att]''
		<dd>
			Represents an element with the <code>att</code> attribute,
			whatever the value of the attribute.

		<dt><css>[att=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute
			whose value is exactly "val".

		<dt><css>[att~=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute
			whose value is a <a href="#whitespace">whitespace</a>-separated list of words,
			one of which is exactly "val".
			If "val" contains whitespace,
			it will never represent anything
			(since the words are <em>separated</em> by spaces).
			Also if "val" is the empty string,
			it will never represent anything.

		<dt><css>[att|=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute,
			its value either being exactly "val"
			or beginning with "val" immediately followed by "-" (U+002D).
			This is primarily intended to allow language subcode matches
			(e.g., the <code>hreflang</code> attribute on the <a element>a</a> element in HTML)
			as described in BCP 47 ([[BCP47]]) or its successor.
			For <code>lang</code> (or <code>xml:lang</code>) language subcode matching,
			please see the '':lang'' pseudo-class.
	</dl>

	Attribute values must be <<ident-token>>s or <<string-token>>s. [[!CSS3SYN]]

	<div class="example">
		Examples:

		The following attribute selector represents an <a element>h1</a> element
		that carries the <code>title</code> attribute,
		whatever its value:

		<pre>h1[title]</pre>

		In the following example, the selector represents a
		<a element>span</a> element whose <code>class</code> attribute has
		exactly the value "example":

		<pre>span[class="example"]</pre>

		Multiple attribute selectors can be used to represent several
		attributes of an element, or several conditions on the same
		attribute. Here, the selector represents a <a element>span</a> element
		whose <code>hello</code> attribute has exactly the value "Cleveland"
		and whose <code>goodbye</code> attribute has exactly the value
		"Columbus":

		<pre>span[hello="Cleveland"][goodbye="Columbus"]</pre>

		The following CSS rules illustrate the differences between
		"=" and "~=".  The first selector would match, for example, an
		<a element>a</a> element with the value "copyright copyleft
		copyeditor" on a <code>rel</code> attribute. The second selector
		would only match an <a element>a</a> element with an <code>href</code>
		attribute having the exact value "http://www.w3.org/".

		<pre>
		a[rel~="copyright"] { ... }
		a[href="http://www.w3.org/"] { ... }
		</pre>

		The following selector represents an <a element>a</a> element
		whose <code>hreflang</code> attribute is exactly "fr".

		<pre>a[hreflang=fr] </pre>

		The following selector represents an <a element>a</a> element for
		which the value of the <code>hreflang</code> attribute begins with
		"en", including "en", "en-US", and "en-scouse":

		<pre>a[hreflang|="en"] </pre>

		The following selectors represent a <a element lt=''>DIALOGUE</a> element
		whenever it has one of two different values for an attribute
		<code>character</code>:

		<pre>
		DIALOGUE[character=romeo]
		DIALOGUE[character=juliet]
		</pre>

	</div>

<h3 id="attribute-substrings">
Substring matching attribute selectors</h3>

	Three additional attribute selectors are provided for matching
	substrings in the value of an attribute:

	<dl>
		<dt><css>[att^=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute
			whose value begins with the prefix "val".
			If "val" is the empty string
			then the selector does not represent anything.

		<dt><css>[att$=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute
			whose value ends with the suffix "val".
			If "val" is the empty string
			then the selector does not represent anything.

		<dt><css>[att*=val]</css>
		<dd>
			Represents an element with the <code>att</code> attribute
			whose value contains at least one instance of the substring "val".
			If "val" is the empty string
			then the selector does not represent anything.
	</dl>

	Attribute values must be <<ident-token>>s or <<string-token>>s.

	<div class="example">
		Examples:
		The following selector represents an HTML <a element>object</a> element,
		referencing an image:

		<pre>object[type^="image/"] </pre>

		The following selector represents an HTML <a element>a</a> element
		with an <code>href</code> attribute whose value ends with ".html".

		<pre>a[href$=".html"] </pre>

		The following selector represents an HTML paragraph
		with a <code>title</code> attribute whose value contains the substring "hello"

		<pre>p[title*="hello"] </pre>
	</div>

<h3 id="attribute-case">
Case-sensitivity</h3>

	By default case-sensitivity of attribute names and values in selectors
	depends on the document language. To match attribute values case-insensitively
	regardless of document language rules, the attribute selector may include the
	identifier <code>i</code> before the closing bracket (<code>]</code>).
	When this flag is present, UAs must match the attribute's value
	case-insensitively within the ASCII range.
	Like the rest of Selectors syntax,
	the <code>i</code> identifier is <a href="#case-sensitive">case-insensitive</a>
	within the ASCII range.

	<div class="example">
		The following rule will style the <code>frame</code> attribute when it
		has a value of <code>hsides</code>, whether that value is represented
		as <code>hsides</code>, <code>HSIDES</code>, <code>hSides</code>, etc.
		even in an XML environment where attribute values are case-sensitive.

		<pre>[frame=hsides i] { border-style: solid none; } </pre>
	</div>

	<!-- plinss notes we may eventually want to choose other normalizations
			 for attribute matching; but since there's no normalization scheme
			 that really seems needed at this point, this issue is deferred to
			 a later level -->

<h3 id="attrnmsp">
Attribute selectors and namespaces</h3>

	The attribute name in an attribute selector is given as a
	<a href="https://www.w3.org/TR/css3-namespace/#css-qnames">CSS qualified
	name</a>: a namespace prefix that has been previously <a href="#nsdecl">declared</a>
	may be prepended to the attribute name separated by the namespace
	separator &quot;vertical bar&quot; (<code>|</code>). In keeping with
	the Namespaces in the XML recommendation, default namespaces do not
	apply to attributes, therefore attribute selectors without a namespace
	component apply only to attributes that have no namespace (equivalent
	to ''|attr''). An asterisk may be used for
	the namespace prefix indicating that the selector is to match all
	attribute names without regard to the attribute's namespace.

	An attribute selector with an attribute name containing a namespace
	prefix that has not been previously <a href="#nsdecl">declared</a> is
	an <a href="#conformance">invalid</a> selector.

	<div class="example">
		CSS examples:

		<pre>
		@namespace foo "http://www.example.com";
		[foo|att=val] { color: blue }
		[*|att] { color: yellow }
		[|att] { color: green }
		[att] { color: green }
		</pre>

		The first rule will match only elements with the attribute
		<code>att</code> in the "http://www.example.com" namespace with the
		value "val".

		The second rule will match only elements with the attribute
		<code>att</code> regardless of the namespace of the attribute
		(including no namespace).

		The last two rules are equivalent and will match only elements
		with the attribute <code>att</code> where the attribute is not
		in a namespace.
	</div>

<h3 id="def-values">
Default attribute values in DTDs</h3>

	Attribute selectors represent attribute values in the document tree.
	How that document tree is constructed is outside the scope of Selectors.
	In some document formats default attribute values can be defined in a DTD or
	elsewhere, but these can only be selected by attribute selectors if they
	appear in the document tree. Selectors should be designed so that they
	work whether or not the default values are included in the document tree.

	For example, a XML UA may, but is <em>not</em> required to,
	read an “external subset” of the DTD, but <em>is</em> required to
	look for default attribute values in the document's “internal subset”.
	(See, e.g., [[XML10]] for definitions of these subsets.)
	Depending on the UA, a default attribute value defined in the external subset of the DTD
	might or might not appear in the document tree.

	A UA that recognizes an XML namespace may, but is not required to use its
	knowledge of that namespace to treat default attribute values as if
	they were present in the document. (For example, an XHTML UA is not
	required to use its built-in knowledge of the XHTML DTD. See, e.g., [[XML-NAMES]] for details on namespaces in XML
	1.0.)

	Note: Typically, implementations
	choose to ignore external subsets. This corresponds to the behavior
	of non-validating processors as defined by the XML specification.

	<div class="example">
		Example:

		Consider an element <code>EXAMPLE</code> with an attribute <code>radix</code>
		that has a default value of <code>"decimal"</code>. The DTD fragment might be

		<pre class="dtd-example">&lt;!ATTLIST EXAMPLE radix (decimal,octal) "decimal"> </pre>

		If the style sheet contains the rules

		<pre>
		EXAMPLE[radix=decimal] { /*... default property settings ...*/ }
		EXAMPLE[radix=octal]   { /*... other settings...*/ }
		</pre>

		the first rule might not match elements whose <code>radix</code> attribute is
		set by default, i.e. not set explicitly. To catch all cases, the
		attribute selector for the default value must be dropped:

		<pre>
		EXAMPLE                { /*... default property settings ...*/ }
		EXAMPLE[radix=octal]   { /*... other settings...*/ }
		</pre>

		Here, because the selector ''EXAMPLE[radix=octal]'' is
		more specific than the type selector alone, the style declarations in
		the second rule will override those in the first for elements that
		have a <code>radix</code> attribute value of <code>"octal"</code>. Care has to be taken that
		all property declarations that are to apply only to the default case
		are overridden in the non-default cases' style rules.
	</div>

<h3 id="class-html">
Class selectors</h3>

	The <dfn export>class selector</dfn> is given as a full stop (. U+002E) immediately
	followed by an identifier. It represents an element belonging to the
	class identified by the identifier, as defined by the document language.
	For example, in [[HTML5]], [[SVG11]], and [[MATHML]] membership in a
	class is given by the <code>class</code> attribute: in these languages
	it is equivalent to the <code>~=</code> notation applied to the
	local <code>class</code> attribute
	(i.e. <code>[class~=<var>identifier</var>]</code>).

	<div class="example">
		CSS examples:

		We can assign style information to all elements with
		<code>class~="pastoral"</code> as follows:

		<pre>*.pastoral { color: green }  /* all elements with class~=pastoral */ </pre>

		or just

		<pre>.pastoral { color: green }  /* all elements with class~=pastoral */ </pre>

		The following assigns style only to H1 elements with
		<code>class~="pastoral"</code>:

		<pre>H1.pastoral { color: green }  /* H1 elements with class~=pastoral */ </pre>

		Given these rules, the first <code>H1</code> instance below would not have
		green text, while the second would:

		<pre>
		&lt;H1>Not green&lt;/H1>
		&lt;H1 class="pastoral">Very green&lt;/H1>
		</pre>

		The following rule matches any <a element>P</a> element whose <code>class</code>
		attribute has been assigned a list of <a
		href="#whitespace">whitespace</a>-separated values that includes both
		<code>pastoral</code> and <code>marine</code>:

		<pre>p.pastoral.marine { color: green } </pre>

		This rule matches when <code>class="pastoral blue aqua
		marine"</code> but does not match for <code>class="pastoral
		blue"</code>.
	</div>

	Note: Because CSS gives considerable
	power to the "class" attribute, authors could conceivably design their
	own "document language" based on elements with almost no associated
	presentation (such as <a element>div</a> and <a element>span</a> in HTML)
	and assigning style
	information through the "class" attribute.  Authors should avoid this
	practice since the structural elements of a document language often
	have recognized and accepted meanings and author-defined classes may
	not.

	Note: If an element has multiple
	class attributes, their values must be concatenated with spaces
	between the values before searching for the class. As of this time the
	working group is not aware of any manner in which this situation can
	be reached, however, so this behavior is explicitly non-normative in
	this specification.

	When matching against a document which is in <a>quirks mode</a>,
	class names must be matched <a lt="ASCII case-insensitive">ASCII case-insensitively</a>;
	class selectors are otherwise case-sensitive.

<h3 id="id-selectors">
ID selectors</h3>

	Document languages may contain attributes that are declared to be of type ID.
	What makes attributes of type ID special
	is that no two such attributes can have the same value in a conformant document,
	regardless of the type of the elements that carry them;
	whatever the document language,
	an ID typed attribute can be used to uniquely identify its element.
	In HTML all ID attributes are named <code>id</code>;
	XML applications may name ID attributes differently,
	but the same restriction applies.
	Which attribute on an element is considered the “ID attribute“ is defined by the document language.

	An <dfn export>ID selector</dfn> consists of a “number sign” (U+0023, <code>#</code>)
	immediately followed by the ID value,
	which must be a CSS <a href="https://www.w3.org/TR/CSS21/syndata.html#value-def-identifier">identifier</a>.
	An ID selector represents an element instance that has an identifier that matches the identifier in the ID selector.
	(It is possible in non-conforming documents for multiple elements to match a single ID selector.)

	<div class="example">
		Examples:
		The following ID selector represents an <a element>h1</a> element
		whose ID-typed attribute has the value "chapter1":

		<pre>h1#chapter1 </pre>

		The following ID selector represents any element whose ID-typed
		attribute has the value "chapter1":

		<pre>#chapter1 </pre>

		The following selector represents any element whose ID-typed
		attribute has the value "z98y".

		<pre>*#z98y </pre>
	</div>

	Note: In XML 1.0 [[XML10]], the information about which attribute
	contains an element's IDs is contained in a DTD or a schema. When
	parsing XML, UAs do not always read the DTD, and thus may not know
	what the ID of an element is (though a UA may have namespace-specific
	knowledge that allows it to determine which attribute is the ID
	attribute for that namespace). If a style sheet author knows or
	suspects that a UA may not know what the ID of an element is, he
	should use normal attribute selectors instead:
	''[name=p371]'' instead of ''#p371''.

	If an element has multiple ID attributes, all of them must be
	treated as IDs for that element for the purposes of the ID
	selector. Such a situation could be reached using mixtures of xml:id,
	DOM3 Core, XML DTDs, and namespace-specific knowledge.

	When matching against a document which is in <a>quirks mode</a>,
	IDs must be matched <a lt="ASCII case-insensitive">ASCII case-insensitively</a>;
	ID selectors are otherwise case-sensitive.

<h2 id="linguistic-pseudos">
Linguistic Pseudo-classes</h2>

<h3 id="the-dir-pseudo">
The Directionality Pseudo-class: '':dir()''</h3>

	The <dfn id='dir-pseudo'>:dir()</dfn> pseudo-class allows the author to write
	selectors that represent an element based on its directionality as
	determined by the document language.
	For example, [[HTML5]] defines <a href="http://www.whatwg.org/html/selectors.html#selector-ltr">how to determine the directionality of an element</a>,
	based on a combination of the <code>dir</code> attribute, the surrounding text, and other factors.
	The '':dir()'' pseudo-class does not select based on stylistic
	states&#8212;for example, the CSS 'direction' property does not affect
	whether it matches.

	The pseudo-class '':dir(ltr)'' represents an element that
	has a directionality of left-to-right (<code>ltr</code>). The
	pseudo-class '':dir(rtl)'' represents an element that has
	a directionality of right-to-left (<code>rtl</code>). The argument to
	'':dir()'' must be a single identifier, otherwise the selector
	is invalid. White space is optionally allowed between the identifier
	and the parentheses. Values other than <code>ltr</code> and
	<code>rtl</code> are not invalid, but do not match anything. (If a
	future markup spec defines other directionalities, then Selectors may
	be extended to allow corresponding values.)

	The difference between '':dir(C)'' and ''[dir=C]''
	is that ''[dir=C]'' only performs a comparison against a given
	attribute on the element, while the '':dir(C)'' pseudo-class
	uses the UAs knowledge of the document's semantics to perform the
	comparison. For example, in HTML, the directionality of an element
	inherits so that a child without a <code>dir</code> attribute will have
	the same directionality as its closest ancestor with a valid <code>dir</code>
	attribute. As another example, in HTML,
	an element that matches ''[dir=auto]'' will match either
	'':dir(ltr)'' or '':dir(rtl)'' depending on the resolved
	directionality of the elements as determined by its contents. [[HTML5]]

<h3 id="the-lang-pseudo">
The Language Pseudo-class: '':lang()''</h3>

	If the document language specifies how the (human) content language of an
	element is determined, it is possible to write selectors that
	represent an element based on its language.
	The <dfn id='lang-pseudo'>:lang()</dfn> pseudo-class represents an element that
	is in one of the languages listed in its argument. It accepts
	a comma-separated list of one or more <a>language ranges</a> as its
	argument. Each <dfn>language range</dfn> in '':lang()''
	must be a valid CSS <<ident>> or <<string>>.
	(Language ranges containing asterisks, for example,
		must be quoted as strings.)

	The <dfn>language</dfn> of an element is defined by the document language.
	For example, in HTML [[HTML401]], the <a>language</a> is determined by a
	combination of the <code>lang</code> attribute, information from
	<a element>meta</a> elements, and possibly also the protocol (e.g.
	from HTTP headers). XML languages can use the <code>xml:lang</code>
	attribute to indicate language information for an element.

	The element's <a>language</a> matches a <a>language range</a> if
	the element's <a>language</a> (normalized to BCP 47 syntax if necessary)
	matches the given <a>language range</a> in an <var>extended filtering</var>
	operation per [[RFC4647]] <cite>Matching of Language Tags</cite> (section 3.3.2).
	The matching is performed case-insensitively within the ASCII range.
	The <a>language range</a> does not need to be a valid language code to
	perform this comparison.

	Note: It is recommended that
	documents and protocols indicate language using codes from BCP 47 [[BCP47]]
	or its successor, and by means of <code>xml:lang</code> attributes in the
	case of XML-based documents [[XML10]]. See <a
	href="http://www.w3.org/International/questions/qa-lang-2or3.html">
	"FAQ: Two-letter or three-letter language codes."</a>

	<div class="example">
		Examples:
		The two following selectors represent an HTML document that is in
		Belgian French or German. The two next selectors represent
		<a element>q</a> quotations in an arbitrary element in Belgian French
		or German.

		<pre>
		html:lang(fr-be)
		html:lang(de)
		:lang(fr-be) > q
		:lang(de) > q
		</pre>
	</div>

	Note: One difference between '':lang(C)'' and the ''|='' operator
	is that the ''|='' operator only performs a comparison against a given
	attribute on the element, while the '':lang(C)'' pseudo-class
	uses the UAs knowledge of the document's semantics to perform the
	comparison.

	<div class=example>
		In this HTML example, only the BODY matches
		''[lang|=fr]'' (because it has a LANG attribute) but both
		the BODY and the P match '':lang(fr)'' (because both are in
		French). The P does not match the ''[lang|=fr]'' because it
		does not have a LANG attribute.

		<pre>
		&lt;body lang=fr>
		  &lt;p>Je suis français.&lt;/p>
		&lt;/body>
		</pre>
	</div>

	<div class=example>
		Another difference between '':lang(C)'' and the ''|='' operator
		is that '':lang(C)'' performs implicit wildcard matching.

		For example, '':lang(de-DE)'' will match all of ''de-DE'',
		''de-DE-1996'', ''de-Latn-DE'', ''de-Latf-DE'', ''de-Latn-DE-1996'',
		whereas of those ''[lang|=de-DE]'' will only match ''de-DE'' and
		''de-DE-1996''.

		To perform wildcard matching on the first subtag (the primary language),
		an asterisk must be used: ''*-CH'' will match all of ''de-CH'',
		''it-CH'', ''fr-CH'', and ''rm-CH''.
	</div>

	Note: Wildcard language matching is new in Level 4.


<h2 id="location">
Location Pseudo-classes</h2>

<h3 id="the-any-link-pseudo">
The Hyperlink Pseudo-class: '':any-link''</h3>

	The <dfn id='any-link-pseudo'>:any-link</dfn> pseudo-class represents an element
	that acts as the source anchor of a hyperlink.
	For example, in [[HTML5]], any <a element>a</a>, <a element>area</a>, or <a element>link</a> elements with an <code>href</code> attribute
	are hyperlinks, and thus <a href="http://www.whatwg.org/html/selectors.html#selector-link">match <code>:any-link</code></a>.
	It matches an element if the element would match '':link'' or '':visited'',
	equivalent to '':matches(:link, :visited)''.


<h3 id="link">
The Link History Pseudo-classes: '':link'' and '':visited''</h3>

	User agents commonly display unvisited <a href="#the-any-link-pseudo">hyperlinks</a> differently from
	previously visited ones. Selectors
	provides the pseudo-classes <dfn id='link-pseudo'>:link</dfn> and
	<dfn id='visited-pseudo'>:visited</dfn> to distinguish them:

	<ul>
		<li>The '':link'' pseudo-class applies to links that have
		not yet been visited.
		<li>The '':visited'' pseudo-class applies once the link has
		been visited by the user.
	</ul>

	After some amount of time, user agents may choose to return a
	visited link to the (unvisited) '':link'' state.

	The two states are mutually exclusive.

	<div class="example">
		The following selector represents links carrying class
		<code>footnote</code> and already visited:

		<pre>.footnote:visited </pre>
	</div>

	Since it is possible for style sheet authors to abuse the :link and :visited pseudo-classes
	to determine which sites a user has visited without the user's consent,
	UAs may treat all links as unvisited links
	or implement other measures to preserve the user's privacy
	while rendering visited and unvisited links differently.

<h3 id="the-target-pseudo">
The Target Pseudo-class: '':target''</h3>

	In some document languages,
	the document's URL can further point to specific elements <em>within</em> the document
	via the URL's <a for=url>fragment</a>.
	The elements pointed to in this way are the target elements of the document.

	<div class='example'>
		In HTML the fragment points to the element in the page with the same ID.
		The url <code>https://example.com/index.html#section2</code>,
		for example,
		points to the element with <code>id="section2"</code>
		in the document at <code>https://example.com/index.html</code>.
	</div>

	The <dfn id='target-pseudo'>:target</dfn> pseudo-class matches the document's target elements.
	If the document's URL has no fragment identifier, then the document has no target elements.

	<div class="example">
		Example:

		<pre>p.note:target </pre>

		This selector represents a <{p}> element of class
		<code>note</code> that is the target element of the referring
		URL.
	</div>

	<div class="example">
		CSS example:
		Here, the '':target'' pseudo-class is used to make the
		target element red and place an image before it, if there is one:

		<pre>
		:target { color : red }
		:target::before { content : url(target.png) }
		</pre>
	</div>


<h3 id="the-scope-pseudo">
The Reference Element Pseudo-class: '':scope''</h3>

	In some contexts, selectors can be matched with an explicit set of <dfn dfn export lt=":scope element">:scope elements</dfn>.
	This is is a (potentially empty) set of elements
	that provide a reference point for selectors to match against,
	such as that specified by the <code>querySelector()</code> call in [[DOM]].

	The <dfn id='scope-pseudo'>:scope</dfn> pseudo-class represents any element that is a <a>:scope element</a>.
	If the <a>:scope elements</a> are not explicitly specified,
	but the selector is <a>scoped</a> and the <a>scoping root</a> is an element,
	then '':scope'' represents the <a>scoping root</a>;
	otherwise, it represents the root of the document
	(equivalent to '':root'').
	Specifications intending for this pseudo-class to match specific elements
	rather than the document's root element
	must define either a <a>scoping root</a> (if using <a>scoped selectors</a>) or an explicit set of <a>:scope elements</a>.

<h2 id="useraction-pseudos">
User Action Pseudo-classes</h2>

	Interactive user agents sometimes change the rendering in response to user actions.
	Selectors provides several pseudo-classes for the selection of an element the user is acting on.
	(In non-interactive user agents, these pseudo-classes are valid, but never match any element.)

	These pseudo-classes are not mutually exclusive. An element may
	match several pseudo-classes at the same time.

	<div class="example">
		Examples:

		<pre>
		a:link    /* unvisited links */
		a:visited /* visited links */
		a:hover   /* user hovers */
		a:active  /* active links */
		</pre>

		An example of combining dynamic pseudo-classes:

		<pre>
		a:focus
		a:focus:hover
		</pre>

		The last selector matches <a element>a</a> elements that are in
		the pseudo-class :focus and in the pseudo-class :hover.
	</div>

	Note: The specifics of hit-testing,
	necessary to know when several of the pseudo-classes defined in this section apply,
	are not yet defined,
	but will be in the future.

<h3 id="the-hover-pseudo">
The Pointer Hover Pseudo-class: '':hover''</h3>

	The <dfn id='hover-pseudo'>:hover</dfn> pseudo-class applies
	while the user designates an element with a pointing device,
	but does not necessarily activate it.
	For example, a visual user agent could apply this pseudo-class
	when the cursor (mouse pointer) hovers over a box generated by the element.
	Interactive user agents that cannot detect hovering due to hardware limitations
	(e.g., a pen device that does not detect hovering)
	are still conforming.

	An element also matches '':hover''
	if one of its descendants in the <a>flat tree</a>
	(including non-element nodes, such as text nodes)
	matches the above conditions.

	Document languages may define additional ways in which an element can match '':hover''.
	For example, [[HTML5]] defines a labeled control element as <a href="http://www.whatwg.org/html/selectors.html#selector-hover">matching <code>:hover</code></a>
	when its <a element>label</a> is hovered.

	Note: Since the ':hover' state can apply to
	an element because its child is designated by a pointing device, then it is
	possible for ':hover' to apply to an element that is not underneath
	the pointing device.

	The '':hover'' pseudo-class can apply to any pseudo-element.

<h3 id="the-active-pseudo">
The Activation Pseudo-class: '':active''</h3>

	The <dfn id='active-pseudo'>:active</dfn> pseudo-class applies while an element
	is being activated by the user. For example, between the times the
	user presses the mouse button and releases it. On systems with more
	than one mouse button, '':active'' applies only to the
	primary or primary activation button (typically the "left" mouse
	button), and any aliases thereof.

	There may be document language or implementation specific limits on
	which elements can become '':active''.
	For example, [[HTML5]] defines a <a href="http://www.whatwg.org/html/selectors.html#selector-active">list of activatable elements</a>.

	An element also matches '':active''
	if one of its descendants in the <a>flat tree</a>
	(including non-element nodes, such as text nodes)
	matches the above conditions.

	Document languages may define additional ways in which an element can match '':active''.

	Note: An element can be both
	':visited' and ':active' (or ':link' and ':active').

<h3 id="the-focus-pseudo">
The Input Focus Pseudo-class: '':focus''</h3>

	The <dfn id='focus-pseudo'>:focus</dfn> pseudo-class applies
	while an element has the focus
	(accepts keyboard or mouse events, or other forms of input).

	There may be document language or implementation specific limits on
	which elements can acquire :focus.
	For example, [[HTML]] defines a list of <a href="https://html.spec.whatwg.org/multipage/interaction.html#focusable-area">focusable areas</a>.

	Document languages may define additional ways in which an element can match '':focus'',
	except that the '':focus'' pseudo class must not automatically propagate to the parent element
	(see '':focus-within'' if matching on the parent is desired).
	Note: it may still apply to the parent element
	if made to propagate due to other mechanisms,
	but not merely due to being the parent.

<h3 id="the-focusring-pseudo">
The Input Focus-Ring Pseudo-class: '':focus-ring''</h3>

	The <dfn id='focus-ring-pseudo'>:focus-ring</dfn> pseudo-class applies
	while an element matches the '':focus'' pseudo-class,
	<em>and</em> the UA determines via heuristics
	that the focus should be specially indicated on the element
	(typically via a "focus ring").

	<div class=example>
		For example, UAs typically display focus indicators on text elements
		<em>whenever</em> they're focused,
		to draw attention to the fact that keyboard input will affect their contents.

		On the other hand, they typically only display focus indicators on buttons
		when they were focused by a keyboard interaction
		(such as tabbing through the document),
		because it's not always immediately obvious where the focus will move
		after such an interaction,
		but not when they were focused by more "obvious" interactions,
		like clicking on the button with a mouse pointer.
	</div>

	<div class=advisement>
		Page authors should follow these guidelines
		when deciding whether to use '':focus'' or '':focus-ring''
		to style the focused state of an element:

		* If the element has "native" focus indicator behavior
			(such as text fields or buttons),
			use '':focus-ring''.
		* Otherwise, if the element is emulating a text input,
			or something else that is <em>intended</em> to receive keyboard interaction,
			use '':focus''.
		* Otherwise,
			use '':focus-ring''.
	</div>

	When UAs choose to specially indicate focus on an element,
	or whether they specially indicate focus at all,
	is UA-dependent.
	Different UAs,
	the same UA on different operating systems,
	or the same UA on the same OS,
	but with different user settings,
	can make different choices as to when an element matches '':focus-ring''.

	<div class=example>
		The following guidelines are suggested heuristics
		for choosing when to apply '':focus-ring''
		to elements without "native" focus ring behavior:

		* If the element received focus via a keyboard interaction,
			including indirectly,
			such as triggering a dialog
			by pressing a button using the keyboard,
			apply '':focus-ring''.
		* If a keyboard event occurs while an element is focused,
			even if the element wasn't focused by a keyboard interaction,
			apply '':focus-ring''.
	</div>

<h3 id="the-focus-within-pseudo">
The Generalized Input Focus Pseudo-class: '':focus-within''</h3>

	The <dfn id='focus-within-pseudo'>:focus-within</dfn> pseudo-class applies to elements for which the '':focus'' pseudo class applies.

	An element also matches '':focus-within''
	if one of its descendants in the <a>flat tree</a>
	(including non-element nodes, such as text nodes)
	matches the conditions for matching '':focus''.

<h3 id="drag-pseudos">
The Drag-and-Drop Pseudo-class: '':drop'' and '':drop()''</h3>

	The <dfn>:drop</dfn> pseudo-class applies to all elements
	that are drop targets,
	as defined by the document language,
	while the user is “dragging”
	or otherwise conceptually carrying an item
	to be “dropped”.
	For example, in HTML the <a href="http://www.whatwg.org/specs/web-apps/current-work/multipage/dnd.html#the-dropzone-attribute"><code>dropzone</code></a> attribute
	specified that an element is a drop target.

	The <dfn id="selectordef-drop-function">:drop()</dfn> functional pseudo-class is identical to '':drop'',
	but allows additional filters to be specified that can exclude some drop targets.
	Its syntax is:

	<pre class='prod'>:drop( [ active || valid || invalid ]? ) </pre>

	The keywords have the following meanings:

	<dl dfn-type="value" dfn-for=":drop()">
		<dt><dfn>active</dfn>
		<dd>
			The drop target is the current drop target for the drag operation.
			That is, if the user were to release the drag,
			it would be dropped onto this drop target.

		<dt><dfn>valid</dfn>
		<dd>
			If the document language has a concept of “valid” and “invalid” drop targets,
			this only matches if the drop target is valid for the object currently being dragged.
			Otherwise, it matches all drop targets.

			For example, HTML's <code>dropzone</code> attribute can specify that the drop target only accepts strings or files that are set to a given type.

		<dt><dfn>invalid</dfn>
		<dd>
			If the document language has a concept of “valid” and “invalid” drop targets,
			this only matches if the drop target is invalid for the object currently being dragged.
			Otherwise, it matches nothing.
	</dl>

	Multiple keywords can be combined in the argument,
	representing only drop targets that satisfy all of the keywords.
	For example, '':drop(valid active)'' will match the active drop target <em>if</em> it's valid,
	but not if it's invalid.

	If no keywords are given in the argument,
	'':drop()'' has the same meaning as '':drop''--
	it matches every drop target.

	Issue: Turn <a href="http://forums.mozillazine.org/viewtopic.php?f=18&t=2633249">this scenario</a> into an example.

<h2 id="time-pseudos">
Time-dimensional Pseudo-classes</h2>

	These pseudo-classes classify elements with respect to the
	currently-displayed or active position in some timeline, such as
	during speech rendering of a document, or during the display of
	a video using WebVTT to render subtitles.

	CSS does not define this timeline;
	the host language must do so.
	If there is no timeline defined for an element,
	these pseudo-classes must not match the element.

	Note: Ancestors of a '':current'' element are also '':current'',
	but ancestors of a '':past'' or '':future'' element are not necessarily '':past'' or '':future'' as well.
	A given element matches at most one of '':current'', '':past'', or '':future''.

<h3 id="the-current-pseudo">
The Current-element Pseudo-class: '':current''</h3>

	The <dfn id='current-pseudo'>:current</dfn> pseudo-class represents the
	element, or an ancestor of the element, that is currently being displayed.

	Its alternate form <dfn>:current()</dfn>, like '':matches()'',
	takes a list of <a>compound selectors</a> as its argument: it represents the
	'':current'' element that matches the argument or, if that does
	not match, the innermost ancestor of the '':current'' element
	that does. (If neither the '':current'' element nor its ancestors
	match the argument, then the selector does not represent anything.)

	<div class="example">
		For example, the following rule will highlight whichever paragraph
		or list item is being read aloud in a speech rendering of the document:

		<pre>
		:current(p, li, dt, dd) {
		  background: yellow;
		}
		</pre>
	</div>

<h3 id="the-past-pseudo">
The Past-element Pseudo-class: '':past''</h3>

	The <dfn id='past-pseudo'>:past</dfn> pseudo-class represents any element that is
	defined to occur entirely prior to a '':current'' element.
	For example, the WebVTT spec defines the '':past'' pseudo-class <a href="http://dev.w3.org/html5/webvtt/#the-past-and-future-pseudo-classes">relative to the current playback position of a media element</a>.
	If a time-based order of elements is not defined by the document language,
	then this represents any element that is a (possibly indirect) previous
	sibling of a '':current'' element.

<h3 id="the-future-pseudo">
The Future-element Pseudo-class: '':future''</h3>

	The <dfn id='future-pseudo'>:future</dfn> pseudo-class represents any element that is
	defined to occur entirely after a '':current'' element.
	For example, the WebVTT spec defines the '':future'' pseudo-class <a href="http://dev.w3.org/html5/webvtt/#the-past-and-future-pseudo-classes">relative to the current playback position of a media element</a>.
	If a time-based order of elements is not defined by the document language,
	then this represents any element that is a (possibly indirect) next
	sibling of a '':current'' element.


<h2 id='resource-pseudos'>
Resource State Pseudos</h2>

	The pseudo-classes in this section apply to elements that represent loaded resources,
	particularly images/videos,
	and allow authors to select them based on some quality of their "state".

<h3 id="video-state">
Video/Audio Play State: the '':playing'' and '':paused'' pseudo-classes</h3>

	The <dfn>:playing</dfn> pseudo-class represents an element
	representing an audio, video, or similar resource
	that is capable of being "played" or "paused",
	when that element is "playing".
	(This includes both when the element is explicitly playing,
	and when it's temporarily stopped for some reason not connected to user intent,
	but will automatically resume when that reason is resolved,
	such as a "buffering" state.)

	The <dfn>:paused</dfn> pseudo-class represents the same elements,
	but instead match when the element is <em>not</em> "playing".
	(This includes both an explicit "paused" state,
	and other non-playing states like "loaded, hasn't been activated yet", etc.)


<h2 id='input-pseudos'>
The Input Pseudo-classes</h2>

	The pseudo-classes in this section mostly apply to elements that take user input,
	such as HTML's <a element>input</a> element.

<h3 id="input-states">
Input Control States</h3>

<h4 id="enableddisabled">
The '':enabled'' and '':disabled'' Pseudo-classes</h4>

	The <dfn id='enabled-pseudo'>:enabled</dfn> pseudo-class represents user interface elements
	that are in an enabled state; such elements have a corresponding disabled
	state.

	Conversely, the <dfn id='disabled-pseudo'>:disabled</dfn> pseudo-class represents user
	interface elements that are in a disabled state; such elements have a
	corresponding enabled state.

	What constitutes an enabled state, a disabled state, and a user interface
	element is host-language-dependent. In a typical document most elements will be
	neither '':enabled'' nor '':disabled''.
	For example, [[HTML5]] defines <a href="http://www.whatwg.org/html/selectors.html#selector-enabled">non-disabled interactive elements</a> to be '':enabled'',
	and any such elements that are <a href="http://www.whatwg.org/html/selectors.html#selector-disabled">explicitly disabled</a> to be '':disabled''.

	Note: CSS properties that might affect a user’s ability
	to interact with a given user interface element do not affect whether it
	matches '':enabled'' or '':disabled''; e.g., the
	'display' and 'visibility' properties have no effect
	on the enabled/disabled state of an element.

<h4 id="rw-pseudos">
The Mutability Pseudo-classes: '':read-only'' and '':read-write''</h4>

	An element matches <dfn id="read-write-pseudo">:read-write</dfn> if it is user-alterable,
	as defined by the document language.
	Otherwise, it is <dfn id="read-only-pseudo">:read-only</dfn>.

	For example, in [[HTML5]] a <a href="http://www.whatwg.org/html/selectors.html#selector-read-only">non-disabled non-readonly <code>&lt;input></code> element</a> is '':read-write'',
	as is any element with the <code>contenteditable</code> attribute set to the true state.

<h4 id="placeholder">
The Placeholder-shown Pseudo-class: '':placeholder-shown''</h4>

	Input elements can sometimes show placeholder text
	as a hint to the user on what to type in.
	See, for example, the <code>placeholder</code> attribute in [[HTML5]].
	The <dfn id="placeholder-shown-pseudo">:placeholder-shown</dfn> pseudo-class
	matches an input element that is showing such placeholder text.

<h4 id="the-default-pseudo">
The Default-option Pseudo-class: '':default''</h4>

	The <dfn id='default-pseudo'>:default</dfn> pseudo-class applies to the one or more UI elements
	that are the default among a set of similar elements.  Typically applies to
	context menu items, buttons and select lists/menus.

	One example is the default submit button among a set of buttons.
	Another example is the default option from a popup menu.
	In a select-many group (such as for pizza toppings), multiple elements can match '':default''.
	For example, [[HTML5]] defines that '':default'' matches
	<a href="http://www.whatwg.org/html/selectors.html#selector-default">the “default button” in a form,
	the initially-selected <code>&lt;option></code>(s) in a <code>&lt;select></code>,
	and a few other elements.</a>

<h3 id="input-value-states">
Input Value States</h3>

<h4 id="checked">
The Selected-option Pseudo-class: '':checked''</h4>

	Radio and checkbox elements can be toggled by the user. Some menu
	items are "checked" when the user selects them. When such elements are
	toggled "on" the <dfn id='checked-pseudo'>:checked</dfn> pseudo-class applies.
	For example, [[HTML5]] defines that <a href="http://www.whatwg.org/html/selectors.html#selector-checked">checked checkboxes, radio buttons, and selected <code>&lt;option></code> elements</a> match '':checked''.

	While the '':checked'' pseudo-class is dynamic in nature,
	and can altered by user action,
	since it can also be based on the presence of semantic attributes in the document
	(such as the <code>selected</code> and <code>checked</code> attributes in [[HTML5]]),
	it applies to all media.

	<div class="example">
		An unchecked checkbox can be selected by using the negation
		pseudo-class:

		<pre>input[type=checkbox]:not(:checked)</pre>
	</div>

<h4 id="indeterminate">
The Indeterminate-value Pseudo-class: '':indeterminate''</h4>

	The <dfn id='indetermine-pseudo'>:indeterminate</dfn> pseudo-class applies to UI elements whose
	value is in an indeterminate state.
	For example, radio and checkbox elements can be toggled between checked and unchecked states,
	but are sometimes in an indeterminate state, neither checked nor unchecked.
	Similarly a progress meter can be in an indeterminate state when the percent completion is unknown.
	For example, [[HTML5]] defines how <a href="http://www.whatwg.org/html/selectors.html#selector-indeterminate">checkboxes</a> can be made to match '':indeterminate''.

	Like the '':checked'' pseudo-class, '':indeterminate''
	applies to all media. Components of a radio-group initialized with no
	pre-selected choice, for example, would be '':indeterminate''
	even in a static display.

<h3 id='ui-validity'>
Input Value-checking</h3>

<h4 id="validity-pseudos">
The Validity Pseudo-classes: '':valid'' and '':invalid''</h4>

	An element is <dfn id="valid-pseudo">:valid</dfn>
	or <dfn id="invalid-pseudo">:invalid</dfn>
	when its contents or value is, respectively,
	valid or invalid with respect to data validity semantics defined by the document language
	(e.g. [[XFORMS11]] or [[HTML5]]).
	An element which lacks data validity semantics is neither '':valid'' nor '':invalid''.

	Note: There is a difference between an element which has no constraints,
	and thus would always be '':valid'',
	and one which has no data validity semantics at all,
	and thus is neither '':valid'' nor '':invalid''.
	In HTML, for example, an <code>&lt;input type="text"></code> element may have no constraints,
	but a <a element>p</a> element has no validity semantics at all,
	and so it never matches either of these pseudo-classes.

<h4 id="range-pseudos">
The Range Pseudo-classes: '':in-range'' and '':out-of-range''</h4>

	The <dfn id="in-range-pseudo">:in-range</dfn> and
	<dfn id="out-of-range-pseudo">:out-of-range</dfn> pseudo-classes
	apply only to elements that have range limitations.  An element is
	'':in-range'' or '':out-of-range'' when the value
	that the element is bound to is in range or out of range with respect
	to its range limits as defined by the document language. An element
	that lacks data range limits or is not a form control is neither
	'':in-range'' nor '':out-of-range''.
	E.g. a slider element with a value of 11 presented as a slider control
	that only represents the values from 1-10 is :out-of-range.  Another
	example is a menu element with a value of "E" that happens to be
	presented in a popup menu that only has choices "A", "B" and "C".

<h4 id="opt-pseudos">
The Optionality Pseudo-classes: '':required'' and '':optional''</h4>

	A form element is <dfn id="required-pseudo">:required</dfn> or
	<dfn id="optional-pseudo">:optional</dfn>
	if a value for it is, respectively, required or optional before the
	form it belongs to can be validly submitted. Elements that are not
	form elements are neither required nor optional.

<h4 id="user-pseudos">
The User-interaction Pseudo-class: '':user-invalid''</h4>

	The <dfn id="user-error-pseudo">:user-invalid</dfn> pseudo-class
	represents an element with incorrect input, but only
	<em>after</em> the user has significantly interacted with it.
	The '':user-error'' pseudo-class
	must match an '':invalid'', '':out-of-range'', or blank-but-'':required'' elements
	between the time the user has attempted to submit the form
	and before the user has interacted again with the form element.
	User-agents may allow it to match such elements at other times,
	as would be appropriate for highlighting an error to the user.
	For example, a UA may choose to have '':user-error'' match
	an '':invalid'' element once the user has typed some text into it
	and changed the focus to another element,
	and to stop matching only after the user has successfully corrected the input.

	<div class='example'>
		For example, the input in the following document fragment
		would match '':invalid'' as soon as the page is loaded
		(because it the initial value violates the max-constraint),
		but it won't match '':user-error'' until the user significantly interacts with the element,
		or attempts to submit the form it's part of.

		<pre>
		&lt;form>
		  &lt;label>
		    Volume:
		    &lt;input name='vol' type=number min=0 max=10 value=11>
		  &lt;/label>
		  ...
		&lt;/form>
		</pre>
	</div>

	Issue: Cross-check with <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/%3A-moz-ui-invalid">'':-moz-ui-invalid''</a>.

	Issue: Add <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/%3A-moz-ui-valid">:-moz-ui-valid</a> as '':user-valid'' per WG resolution.

	Issue: Evaluate proposed <a href="https://lists.w3.org/Archives/Public/www-style/2014Feb/0511.html">:dirty pseudo-class</a>

	Issue: Clarify that this (and '':invalid''/'':valid'') can apply to form and fieldset elements.

<h2 id="structural-pseudos">
Tree-Structural pseudo-classes</h2>

	Selectors introduces the concept of <dfn id="structural-pseudo-classes">structural pseudo-classes</dfn>
	to permit selection based on extra information that lies in
	the document tree but cannot be represented by other simple selectors or
	combinators.

	Standalone text and other non-element
	nodes are not counted when calculating the position of an element in the list
	of children of its parent. When calculating the position of an element in
	the list of children of its parent, the index numbering starts at 1.

	The <a>structural pseudo-classes</a> only apply to elements in the document tree;
	they must never match <a>pseudo-elements</a>.


<h3 id="the-root-pseudo">
'':root'' pseudo-class</h3>

	The <dfn id='root-pseudo'>:root</dfn> pseudo-class represents an element that is
	the root of the document.

	For example, in a DOM document,
	the '':root'' pseudo-class matches the root element of the <a interface>Document</a> object.
	In HTML, this would be the <a element>html</a> element
	(unless scripting has been used to modify the document).

<h3 id="the-empty-pseudo">
'':empty'' pseudo-class</h3>

	The <dfn id='empty-pseudo'>:empty</dfn> pseudo-class represents an element that has no children at all.
	In terms of the document tree,
	only element nodes and content nodes
	(such as [[DOM]] text nodes, and entity references)
	whose data has a non-zero length must be considered as affecting emptiness;
	comments, processing instructions, and other nodes
	must not affect whether an element is considered empty or not.

	<div class="example">
		Examples:
		''p:empty'' is a valid representation of the following fragment:

		<pre>&lt;p>&lt;/p></pre>

		''foo:empty'' is not a valid representation for the
		following fragments:

		<pre>&lt;foo>bar&lt;/foo></pre>

		<pre>&lt;foo>&lt;bar>bla&lt;/bar>&lt;/foo></pre>

		<pre>&lt;foo>this is not &lt;bar>:empty&lt;/bar>&lt;/foo></pre>
	</div>

<h3 id='the-blank-pseudo'>
'':blank'' pseudo-class</h3>

	The <dfn id='blank-pseudo'>:blank</dfn> pseudo-class is like the '':empty'' pseudo-class,
	except that it additionally matches elements that only contain
	<a href="https://www.w3.org/TR/css3-text/#white-space-rules">code points affected by whitespace processing</a>. [[!CSS3TEXT]]

	<div class='example'>
		For example, the following element matches '':blank'',
		but not '':empty'',
		because it contains at least one linebreak, and possibly other whitespace:

		<pre>&lt;p><br>&lt;/p></pre>
	</div>

	Issue: We don't like the name '':blank''--
	it doesn't clearly convey a difference from '':empty''.
	Moz uses '':-moz-whitespace-only'',
	which is clear but a little wordy.
	Any other suggestions?


<h3 id='child-index'>
Child-indexed Pseudo-classes</h3>

	The pseudo-classes defined in this section select elements
	based on their index amongst their <a>inclusive siblings</a>.

	Note: Selectors 3 described these selectors as selecting elements based on their index in the child list of their parents.
	(This description survives in the name of this very section, and the names of several of the pseudo-classes.)
	As there was no reason to exclude them from matching elements without parents,
	or with non-element parents,
	they have been rephrased to refer to an element's relative index amongst its siblings.

<h4 id="the-nth-child-pseudo">
'':nth-child()'' pseudo-class</h4>

	The <dfn id='nth-child-pseudo' lt=":nth-child()">:nth-child(<var>An+B</var> [of <var>S</var>]? )</dfn>
	pseudo-class notation represents elements that
	are among <var>An+B</var>th elements
	from the list composed of
	their <a>inclusive siblings</a> that match the <a>selector list</a> <var>S</var>.

	The <var>An+B</var> notation and its interpretation
	are defined in [[css-syntax-3#anb-microsyntax]].
	If <var>S</var> is omitted,
	it defaults to ''*''.

	For example, this selector could address every other row in a table,
	and could be used to alternate the color of paragraph text in a cycle of four.

	<div class="example">
		Examples:

		<pre>
		:nth-child(even)   /* represents the 2nd, 4th, 6th, etc elements
		:nth-child(10n-1)  /* represents the 9th, 19th, 29th, etc elements */
		:nth-child(10n+9)  /* Same */
		:nth-child(10n+-1) /* Syntactically invalid, and would be ignored */
		</pre>
	</div>

	<div class="example">
		By passing a selector argument,
		we can select the Nth element that matches that selector.
		For example, the following selector matches the first three “important” list items,
		denoted by the ''.important'' class:

		<pre>:nth-child(-n+3 of li.important)</pre>

		Note that this is different from moving the selector outside of the function, like:

		<pre>li.important:nth-child(-n+3)</pre>

		This selector instead just selects the first three children
		if they also happen to be "important" list items.
	</div>

	<div class="example">
		Here's another example of using the selector argument,
		to ensure that zebra-striping a table works correctly.

		Normally, to zebra-stripe a table's rows,
		an author would use CSS similar to the following:

		<pre>
		tr {
			background: white;
		}
		tr:nth-child(even) {
			background: silver;
		}
		</pre>

		However, if some of the rows are hidden and not displayed,
		this can break up the pattern,
		causing multiple adjacent rows to have the same background color.
		Assuming that rows are hidden with the ''[hidden]'' attribute in HTML,
		the following CSS would zebra-stripe the table rows robustly,
		maintaining a proper alternating background
		regardless of which rows are hidden:

		<pre>
		tr {
			background: white;
		}
		tr:nth-child(even of :not([hidden])) {
			background: silver;
		}
		</pre>
	</div>

<h4 id="the-nth-last-child-pseudo">
'':nth-last-child()'' pseudo-class</h4>

	The <dfn id='nth-last-child-pseudo' lt=":nth-last-child()">:nth-last-child(<var>An+B</var> [of <var>S</var>]? )</dfn>
	pseudo-class notation represents elements that
	are among <var>An+B</var>th elements
	from the list composed of
	their <a>inclusive siblings</a> that match the <a>selector list</a> <var>S</var>,
	counting backwards from the end.

	The CSS Syntax Module [[!CSS3SYN]] defines the <a href="https://drafts.csswg.org/css-syntax/#anb"><var>An+B</var> notation</a>.
	If <var>S</var> is omitted,
	it defaults to ''*''.

	<div class="example">
		Examples:

		<pre>
		tr:nth-last-child(-n+2)    /* represents the two last rows of an HTML table */

		foo:nth-last-child(odd)    /* represents all odd foo elements in their parent element,
		                              counting from the last one */
		</pre>
	</div>

<h4 id="the-first-child-pseudo">
'':first-child'' pseudo-class</h4>

	The <dfn id='first-child-pseudo'>:first-child</dfn> pseudo-class
	represents an element that if first among its <a>inclusive siblings</a>.
	Same as '':nth-child(1)''.

	<div class="example">
		Examples:
		The following selector represents a <a element>p</a> element that is
		the first child of a <a element>div</a> element:

		<pre>div > p:first-child</pre>

		This selector can represent the <code>p</code> inside the
		<code>div</code> of the following fragment:

		<pre>
		&lt;p> The last P before the note.&lt;/p>
		&lt;div class="note">
			 &lt;p> The first P inside the note.&lt;/p>
		&lt;/div>
		</pre>

		but cannot represent the second <code>p</code> in the following fragment:

		<pre>
		&lt;p> The last P before the note.&lt;/p>
		&lt;div class="note">
			 &lt;h2> Note &lt;/h2>
			 &lt;p> The first P inside the note.&lt;/p>
		&lt;/div>
		</pre>

		The following two selectors are usually equivalent:

		<pre>
		* > a:first-child /* a is first child of any element */
		a:first-child /* Same (assuming a is not the root element) */
		</pre>
	</div>

<h4 id="the-last-child-pseudo">
'':last-child'' pseudo-class</h4>

	The <dfn id='last-child-pseudo'>:last-child</dfn> pseudo-class
	represents an element that is last among its <a>inclusive siblings</a>.
	Same as '':nth-last-child(1)''.

	<div class="example">
		Example:
		The following selector represents a list item <code>li</code> that
		is the last child of an ordered list <code>ol</code>.

		<pre>ol > li:last-child</pre>
	</div>

<h4 id="the-only-child-pseudo">
'':only-child'' pseudo-class</h4>

	The <dfn id='only-child-pseudo'>:only-child</dfn> pseudo-class
	represents an element that has no siblings.
	Same as '':first-child:last-child''
	or '':nth-child(1):nth-last-child(1)'',
	but with a lower specificity.


<h3 id='typed-child-index'>
Typed Child-indexed Pseudo-classes</h3>

	The pseudo-elements in this section are similar to the <a href="#child-index">Child Index Pseudo-classes</a>,
	but they resolve based on an element's index <strong>among elements of the same <a href="#type-selectors">type (tag name)</a></strong> in their sibling list.

<h4 id="the-nth-of-type-pseudo">
'':nth-of-type()'' pseudo-class</h4>

	The <dfn id='nth-of-type-pseudo' lt=":nth-of-type()">:nth-of-type(<var>An+B</var>)</dfn> pseudo-class notation
	represents the same elements that would be matched by '':nth-child(|An+B| of |S|)'',
	where |S| is a [=type selector=] and namespace prefix matching the element in question.
	For example,
	when considering whether an HTML <{img}> element matches this [=pseudo-class=],
	the |S| in question is ''html|img''
	(assuming an appropriate <code>html</code> namespace is declared).

	<div class="example">
		CSS example:
		This allows an author to alternate the position of floated images:

		<pre>
		img:nth-of-type(2n+1) { float: right; }
		img:nth-of-type(2n) { float: left; }
		</pre>
	</div>

	Note: If the type of the element is known ahead of time,
	this pseudo-class is equivalent to using '':nth-child()'' with a type selector.
	That is, ''img:nth-of-type(2)''
	is equivalent to ''*:nth-child(2 of img)''.


<h4 id="the-nth-last-of-type-pseudo">
'':nth-last-of-type()'' pseudo-class</h4>

	The <dfn id='nth-last-of-type-pseudo' lt=":nth-last-of-type()">:nth-last-of-type(<var>An+B</var>)</dfn> pseudo-class notation
	represents the same elements that would be matched by '':nth-last-child(|An+B| of |S|)'',
	where |S| is a [=type selector=] and namespace prefix matching the element in question.
	For example,
	when considering whether an HTML <{img}> element matches this [=pseudo-class=],
	the |S| in question is ''html|img''
	(assuming an appropriate <code>html</code> namespace is declared).

	<div class="example">
		Example:
		To represent all <code>h2</code> children of an XHTML
		<code>body</code> except the first and last, one could use the
		following selector:

		<pre>body > h2:nth-of-type(n+2):nth-last-of-type(n+2) </pre>

		In this case, one could also use '':not()'', although the
		selector ends up being just as long:

		<pre>body > h2:not(:first-of-type):not(:last-of-type) </pre>
	</div>

<h4 id="the-first-of-type-pseudo">
'':first-of-type'' pseudo-class</h4>

	The <dfn id='first-of-type-pseudo'>:first-of-type</dfn> pseudo-class
	represents the same element as '':nth-of-type(1)''.

	<div class="example">
		Example:
		The following selector represents a definition title
		<code>dt</code> inside a definition list <code>dl</code>, this
		<code>dt</code> being the first of its type in the list of children of
		its parent element.

		<pre>dl dt:first-of-type</pre>

		It is a valid description for the first two <code>dt</code>
		elements in the following example but not for the third one:

		<pre>
		&lt;dl>
		  &lt;dt>gigogne&lt;/dt>
		  &lt;dd>
		    &lt;dl>
		      &lt;dt>fus&eacute;e&lt;/dt>
		      &lt;dd>multistage rocket&lt;/dd>
		      &lt;dt>table&lt;/dt>
		      &lt;dd>nest of tables&lt;/dd>
		    &lt;/dl>
		  &lt;/dd>
		&lt;/dl>
		</pre>
	</div>

<h4 id="the-last-of-type-pseudo">
'':last-of-type'' pseudo-class</h4>

	The <dfn id='last-of-type-pseudo'>:last-of-type</dfn> pseudo-class
	represents the same element as '':nth-last-of-type(1)''.

	<div class="example">
		Example:
		The following selector represents the last data cell
		<code>td</code> of a table row <code>tr</code>.

		<pre>tr > td:last-of-type</pre>
	</div>

<h4 id="the-only-of-type-pseudo">
'':only-of-type'' pseudo-class</h4>

	The <dfn id='only-of-type-pseudo'>:only-of-type</dfn> pseudo-class
	represents the same element as '':first-of-type:last-of-type''.


<h2 id="combinators">
Combinators</h2>

<h3 id="descendant-combinators">
Descendant combinator (<code> </code>) <em>or</em> (<code>>></code>)</h3>

	At times, authors may want selectors to describe an element that is
	the descendant of another element in the document tree (e.g., "an
	<a element>em</a> element that is contained within an <a element>H1</a> element").
	The <dfn export>descendant combinator</dfn> expresses such a relationship.
	It has two syntactic forms:
	<ol>
	  <li><a href="#whitespace">whitespace</a> that separates two <a>compound selectors</a> <em>or</em>
	  <li>A doubled child selector (<dfn selector id=selectordef-descendant>>></dfn>)
	</ol>

	A selector of the form ''A B'' or ''A >> B''' represents an element <code>B</code> that is an
	arbitrary descendant of some ancestor element <code>A</code>.

	<div class="example">
		Examples:
		For example, consider the following selector:

		<pre>h1 em</pre>

		It represents an <a element>em</a> element being the descendant of
		an <a element>h1</a> element. It is a correct and valid, but partial,
		description of the following fragment:

		<pre>
		&lt;h1>This &lt;span class="myclass">headline
		is &lt;em>very&lt;/em> important&lt;/span>&lt;/h1>
		</pre>

		The following selector:

		<pre>div * p</pre>

		represents a <a element>p</a> element that is a grandchild or later
		descendant of a <a element>div</a> element. Note the whitespace on
		either side of the "*" is not part of the universal selector; the
		whitespace is a combinator indicating that the <code>div</code> must be the
		ancestor of some element, and that that element must be an ancestor
		of the <code>p</code>.
		The following selector, which combines descendant combinators and
		<a href="#attribute-selectors">attribute selectors</a>, represents an
		element that (1) has the <code>href</code> attribute set and (2) is
		inside a <code>p</code> that is itself inside a <code>div</code>:

		<pre>div p *[href]</pre>
	</div>

	Note: Note that the whitespace form has existed since CSS Level 1,
	whereas the <code>>></code> form is introduced in Level 4.
	It was introduced to give the descendant combinator a visible, non-whitespace form
	and to bridge the gap between the <a>child combinator</a> (<code>></code>)
	and the shadow-piercing descendant combinator (<code>>>></code>) [[CSS-SCOPING-1]].

<h3 id="child-combinators">
Child combinator (<code>></code>)</h3>

	A <dfn export>child combinator</dfn> describes a childhood relationship
	between two elements. A child combinator is made of the
	&quot;greater-than sign&quot; (U+003E, <dfn selector id=selectordef-child>></dfn>) code point and
	separates two <a>compound selectors</a>.

	<div class="example">
		Examples:
		The following selector represents a <a element>p</a> element that is
		child of <code>body</code>:

		<pre>body > p</pre>

		The following example combines descendant combinators and child
		combinators.

		<pre>div ol>li p</pre>
		<!-- LEAVE THOSE SPACES OUT! see below -->

		It represents a <a element>p</a> element that is a descendant of an
		<a element>li</a> element; the <a element>li</a> element must be the
		child of an <a element>ol</a> element; the <a element>ol</a> element must
		be a descendant of a <code>div</code>. Notice that the optional white
		space around the ">" combinator has been left out.
	</div>

	For information on selecting the first child of an element,
	please see the section on the '':first-child'' pseudo-class above.

<h3 id="adjacent-sibling-combinators">
Next-sibling combinator (<code>+</code>)</h3>

	The <dfn export id="next-sibling-combinator">next-sibling combinator</dfn> is made of the “plus sign”
	(U+002B, <dfn selector id=selectordef-adjacent>+</dfn>) code point that separates two
	<a>compound selectors</a>.
	The elements represented by the two <a>compound selectors</a>
	share the same parent in the document tree
	and the element represented by the first <a>compound selector</a>
	immediately precedes the element represented by the second one.
	Non-element nodes (e.g. text between elements)
	are ignored when considering the adjacency of elements.

	<div class="example">
		Examples:
		The following selector represents a <a element>p</a> element
		immediately following a <a element>math</a> element:

		<pre>math + p</pre>

		The following selector is conceptually similar to the one in the
		previous example, except that it adds an attribute selector &mdash; it
		adds a constraint to the <a element>h1</a> element, that it must have
		<code>class="opener"</code>:

		<pre>h1.opener + h2</pre>
	</div>


<h3 id="general-sibling-combinators">
Subsequent-sibling combinator (<code>~</code>)</h3>

	The <dfn export id="subsequent-sibling-combinator">subsequent-sibling combinator</dfn> is made of the &quot;tilde&quot;
	(U+007E, <dfn selector id=selectordef-sibling>~</dfn>) code point that separates two <a>compound selectors</a>.
	The elements represented by the two <a>compound selectors</a> share
	the same parent in the document tree and the element represented by
	the first compound selector precedes (not necessarily immediately) the element
	represented by the second one.

	<div class="example">
		<pre>h1 ~ pre</pre>

		represents a <a element>pre</a> element following an <code>h1</code>. It
		is a correct and valid, but partial, description of:

		<pre>
		&lt;h1>Definition of the function a&lt;/h1>
		&lt;p>Function a(x) has to be applied to all figures in the table.&lt;/p>
		&lt;pre>function a(x) = 12x/13.5&lt;/pre>
		</pre>
	</div>


<h2 id="table-pseudos">
Grid-Structural Selectors</h2>

	The double-association of a cell in a 2D grid (to its row and column)
	cannot be represented by parentage in a hierarchical markup language.
	Only one of those associations can be represented hierarchically: the
	other must be explicitly or implicitly defined in the document language
	semantics. In both HTML and DocBook, two of the most common hierarchical
	markup languages, the markup is row-primary (that is, the row associations
	are represented hierarchically); the columns must be implied.
	To be able to represent such implied column-based relationships, the
	<a>column combinator</a> and the
	'':nth-column()'' and '':nth-last-column()'' pseudo-classes
	are defined.
	In a column-primary format, these pseudo-classes match against row associations instead.

<h3 id="the-column-combinator">
Column combinator</h3>

	The <dfn export>column combinator</dfn>, which consists of two pipes (<dfn selector id="selectordef-column">||</dfn>)
	represents the relationship of a column element
	to a cell element belonging to the column it represents.
	Column membership is determined based on the semantics of the document language only:
	whether and how the elements are presented is not considered.
	If a cell element belongs to more than one column,
	it is represented by a selector indicating membership in any of those columns.

	<div class="example">
		The following example makes cells C, E, and G gray.

		<pre>
		col.selected || td {
			background: gray;
			color: white;
			font-weight: bold;
		}
		</pre>

		<pre>
		&lt;table>
			&lt;col span="2">
			&lt;col class="selected">
			&lt;tr>&lt;td>A &lt;td>B &lt;td>C
			&lt;tr>&lt;td colspan="2">D &lt;td>E
			&lt;tr>&lt;td>F &lt;td colspan="2">G
		&lt;/table>
		</pre>
	</div>

<h3 id="the-nth-column-pseudo">
'':nth-column()'' pseudo-class</h3>

	The <dfn id='nth-column-pseudo' lt=":nth-column()">:nth-column(<var>An+B</var>)</dfn>
	pseudo-class notation represents a cell element belonging to a column
	that has <var>An+B</var>-1 columns
	<strong>before</strong> it, for any positive
	integer or zero value of <code>n</code>. Column membership is determined
	based on the semantics of the document language only: whether and how the
	elements are presented is not considered. If a cell element belongs to
	more than one column, it is represented by a selector indicating any of
	those columns.

	The CSS Syntax Module [[!CSS3SYN]] defines the <a href="https://drafts.csswg.org/css-syntax/#anb"><var>An+B</var> notation</a>.



<h3 id="the-nth-last-column-pseudo">
'':nth-last-column()'' pseudo-class</h3>

	The <dfn id='nth-last-column-pseudo' lt=":nth-last-column()">:nth-last-column(<var>An+B</var>)</dfn>
	pseudo-class notation represents a cell element belonging to a column
	that has <var>An+B</var>-1 columns
	<strong>after</strong> it, for any positive
	integer or zero value of <code>n</code>. Column membership is determined
	based on the semantics of the document language only: whether and how the
	elements are presented is not considered. If a cell element belongs to
	more than one column, it is represented by a selector indicating any of
	those columns.

	The CSS Syntax Module [[!CSS3SYN]] defines the <a href="https://drafts.csswg.org/css-syntax/#anb"><var>An+B</var> notation</a>.

<h2 id="specificity-rules">
Calculating a selector's specificity</h2>

	A selector's <dfn export>specificity</dfn> is calculated for a given element as follows:

	<ul>
		<li>count the number of ID selectors in the selector (= <var>A</var>)
		<li>count the number of class selectors, attributes selectors, and pseudo-classes in the selector (= <var>B</var>)
		<li>count the number of type selectors and pseudo-elements in the selector (= <var>C</var>)
		<li>ignore the universal selector
	</ul>

	If the selector is a <a>selector list</a>,
	this number is calculated for each selector in the list,
	and the specificity of the entire selector is the largest of any individual selector in the list that matches the element.

	A few pseudo-classes provide "evaluation contexts" for other selectors,
	and so have their specificity defined by their contents and how they match:

	<ul>
		<li>
			The specificity of a '':matches()'' pseudo-class is replaced by
			the specificity of its selector list argument.
			(The full selector's specificity is equivalent to expanding out all the combinations in full, without '':matches()''.)
		<li>
			The specificity of an '':is()'' pseudo-class is replaced by 0.
		<li>
			Similarly, the specificity of an '':nth-child()'', '':nth-last-child()'', '':nth-of-type()'', or '':nth-last-of-type()'' selector
		  is the specificity of the pseudo class itself (counting as one pseudo-class selector)
		  plus the specificity of its selector list argument (if any).
		<li>
			The specificity of a '':not()'' pseudo-class is replaced by
		 	the specificity of the most specific complex selector in its selector list argument.
	</ul>

	<div class="example">
		For example:
		<ul>
			<li>
				'':matches(em, #foo)'' has
				a specificity of (0,0,1)--like a tag selector--when matched against <code>&lt;em></code>,
				and a specificity of (1,0,0)--like an ID selector--when matched against  <code>&lt;em id=foo></code>.
			<li>
				'':nth-child(even of li, .item)'' has
				a specificity of (0,1,1)--like a tag selector plus a pseudo-class--when matched against <code>&lt;li></code>,
				and a specificity of (0,2,0)--like a class selector plus a pseudo-class--when matched against  <code>&lt;li class=item id=foo></code>.
			<li>
				'':not(em, #foo)'' has
				a specificity of (1,0,0)--like an ID selector--whenever it matches any element.
		</ul>
	</div>

	Specificities are compared by comparing the three components in order:
	the specificity with a larger <var>A</var> value is more specific;
	if the two <var>A</var> values are tied,
	then the specificity with a larger <var>B</var> value is more specific;
	if the two <var>B</var> values are also tied,
	then the specificity with a larger <var>C</var> value is more specific;
	if all the values are tied,
	the two specificities are equal.

	Due to storage limitations,
	implementations may have limitations on the size of <var>A</var>, <var>B</var>, or <var>C</var>.
	If so, values higher than the limit must be clamped to that limit,
	and not overflow.

	<div class="example">
		Examples:
		<pre>
		*               /* a=0 b=0 c=0 */
		LI              /* a=0 b=0 c=1 */
		UL LI           /* a=0 b=0 c=2 */
		UL OL+LI        /* a=0 b=0 c=3 */
		H1 + *[REL=up]  /* a=0 b=1 c=1 */
		UL OL LI.red    /* a=0 b=1 c=3 */
		LI.red.level    /* a=0 b=2 c=1 */
		#x34y           /* a=1 b=0 c=0 */
		#s12:not(FOO)   /* a=1 b=0 c=1 */
		.foo :matches(.bar, #baz)
		                /* Either a=1 b=1 c=0
		                   or a=0 b=2 c=0, depending
		                   on the element being matched. */
		</pre>
	</div>

	Note: Repeated occurrences of the
	same simple selector are allowed and do increase specificity.

	Note: The specificity of the styles
	specified in an HTML <code>style</code> attribute <a href="https://www.w3.org/TR/css-style-attr/#interpret">is described in CSS Style Attributes</a>. [[CSSSTYLEATTR]]

<h2 id="grammar" oldids="formal-syntax">
Grammar</h2>

	Selectors are [=CSS/parsed=] according to the following grammar:

	<pre class=prod>
	<dfn>&lt;selector-list></dfn> = <<complex-selector-list>>

	<dfn>&lt;complex-selector-list></dfn> = <<complex-selector>>#

	<dfn>&lt;compound-selector-list></dfn> = <<compound-selector>>#

	<dfn>&lt;simple-selector-list></dfn> = <<simple-selector>>#

	<dfn>&lt;relative-selector-list></dfn> = <<relative-selector>>#

	<dfn>&lt;complex-selector></dfn> = <<compound-selector>> [ <<combinator>>? <<compound-selector>> ]*

	<dfn>&lt;relative-selector></dfn> = <<combinator>>? <<complex-selector>>

	<dfn>&lt;combinator></dfn> = '>>' | '>' | '+' | '~' | [ '|' '|' ]

	<dfn>&lt;compound-selector></dfn> = <<simple-selector>>+

	<dfn>&lt;simple-selector></dfn> = <<type-selector>> | <<id-selector>> |
	                    <<class-selector>> | <<attribute-selector>> |
	                    <<pseudo-class-selector>> | <<pseudo-element-selector>>

	<dfn>&lt;type-selector></dfn> = <<wq-name>> | <<ns-prefix>>? '*'

	<dfn>&lt;ns-prefix></dfn> = [ <<ident-token>> | '*' ]? '|'

	<dfn>&lt;wq-name></dfn> = <<ns-prefix>>? <<ident-token>>

	<dfn>&lt;id-selector></dfn> = <<hash-token>>

	<dfn>&lt;class-selector></dfn> = '.' <<ident-token>>

	<dfn>&lt;attribute-selector></dfn> = '[' <<wq-name>> ']' |
	                       '[' <<wq-name>> <<attr-matcher>> [ <<string-token>> | <<ident-token>> ] <<attr-modifier>>? ']'

	<dfn>&lt;attr-matcher></dfn> = [ '~' | '|' | '^' | '$' | '*' ]? '='

	<dfn>&lt;attr-modifier></dfn> = i

	<dfn>&lt;pseudo-class-selector></dfn> = ':' <<ident-token>> |
	                          ':' <<function-token>> <<any-value>> ')'

	<dfn>&lt;pseudo-element-selector></dfn> = ':' <<pseudo-class-selector>>
	</pre>

	In addition to the above grammar,
	the following rules apply:

	* A <<compound-selector>> can contain at most one <<type-selector>>,
		and if it does,
		that must be the first <<simple-selector>> in it.
	* A <<compound-selector>> does not allow any whitespace
		between its constituent <<simple-selector>>s.
	* No whitespace is allowed between the constituents of a
		<<type-selector>>,
		<<class-selector>>,
		<<wq-name>>,
		or <<ns-prefix>>,
		or between the initial punctuation and second token
		of <<pseudo-class-selector>>s and <<pseudo-element-selector>>s.
	* Some older <a>pseudo-elements</a>
		(''::before'', ''::after'', ''::first-line'', and ''::first-letter'')
		can, for legacy reasons,
		be written using the <<pseudo-class-selector>> grammar,
		with only a single ":" character at their start.
	* In <<id-selector>>, the <<hash-token>>’s value must be an <a>identifier</a>.

	Note: The grammar above states that a combinator is optional
	between two <<compound-selector>>s in a <<complex-selector>>.
	This is only for grammatical purposes,
	as the <a>CSS Value Definition Syntax</a>'s lax treatment of whitespace
	makes it difficult to indicate that a grammar term can <em>be</em> whitespace.
	"Omitting" a combinator is actually just specifying the <a>descendant combinator</a>.

	Note: While, per the grammar,
	<a>pseudo-elements</a> can appear anywhere in a <<compound-selector>>,
	and a <<compound-selector>> with a <<pseudo-element-selector>>
	can appear anywhere in a <<complex-selector>>,
	in practice the <<pseudo-element-selector>> will only match anything
	if placed at the end of the last <<compound-selector>>
	in a <<complex-selector>>.
	In some circumstances,
	it can be followed by more <<pseudo-element-selector>>s or <<pseudo-class-selector>>s
	in the same <<compount-selector>>,
	but these are specified on a case-by-case basis.
	(For example, the <a>user-action pseudo-classes</a> are allowed after any <a>pseudo-element</a>,
	and the <a>tree-abiding pseudo-elements</a>
	are allowed after the ''::slotted()'' pseudo-element)



<h2 id='api-hooks'>
API Hooks</h2>

	To aid in the writing of specs that use Selectors concepts,
	this section defines several API hooks that can be invoked by other specifications.

<h3 id='parse-selector' algorithm>
Parse A Selector</h3>

	This section defines how to <dfn export>parse a selector</dfn> from a string <var>source</var>.
	It returns either a complex selector list,
	or failure.

	<ol>
		<li>
			Let <var>selector</var> be the result of parsing <var>source</var> as a <<selector-list>>.
			If it does not match the grammar,
			return failure.

		<li>
			Otherwise,
			if any simple selectors in <var>selector</var> are not recognized by the user agent,
			or <var>selector</var> is otherwise invalid in some way
			(such as, for example, containing an undeclared namespace prefix),
			return failure.

		<li>
			Otherwise,
			return <var>selector</var>.
	</ol>

<h3 id='parse-relative-selector' algorithm>
Parse A Relative Selector</h3>

	This section defines how to <dfn export>parse a relative selector</dfn> from a string <var>source</var>,
	against <a>:scope elements</a> <var>refs</var>.
	It returns either a complex selector list,
	or failure.

	<ol>
		<li>
			Let <var>selector</var> be the result of parsing <var>source</var> as a <<relative-selector-list>>.
			If it does not match the grammar,
			return failure.

		<li>
			Otherwise,
			if any simple selectors in <var>selector</var> are not recognized by the user agent,
			or <var>selector</var> is otherwise invalid in some way
			(such as, for example, containing an undeclared namespace prefix),
			return failure.

		<li>
			Otherwise,
			<a lt="absolutize a relative selector list">absolutize <var>selector</var></a> with <var>refs</var> as the <a>:scope elements</a>.

		<li>
			Return <var>selector</var>.
	</ol>

<h3 id='match-against-element' algorithm>
Match a Selector Against an Element</h3>

	This section defines how to <dfn export>match a selector against an element</dfn>.

	APIs using this algorithm must provide a <var>selector</var> and an <var>element</var>.

	Callers may optionally provide:

	<ul>
		<li>
			a set of <a>:scope elements</a>,
			for resolving the '':scope'' pseudo-class against.
			If not specified,
			the set defaults to being empty.

			Issue: Should it instead default to the root element,
			to match the definition of '':scope''?

			If the selector is a <a>relative selector</a>,
			the set of <a>:scope elements</a> must not be empty.
	</ul>

	This algorithm returns either success or failure.

	For each <a>complex selector</a> in the given <var>selector</var>
	(which is taken to be a <a>list of complex selectors</a>),
	match the complex selector against <var>element</var>,
	as described in the following paragraph.
	If the matching returns success for any complex selector,
	then the algorithm return success; otherwise it returns failure.

	To <dfn>match a complex selector against an element</dfn>,
	process it <a>compound selector</a> at a time,
	in right-to-left order.
	This process is defined recursively as follows:

	<ul>
		<li>If any simple selectors in the rightmost compound selector
		does not match the element, return failure.

		<li>Otherwise, if there is only one compound selector in the
		complex selector, return success.

		<li>Otherwise, consider all possible elements
		that could be related to this element by the rightmost <a>combinator</a>.
		If the operation of matching the selector consisting of this selector
		with the rightmost compound selector and rightmost combinator removed
		against any one of these elements returns success, then return success.
		Otherwise, return failure.
	</ul>

<h3 id='match-against-pseudo-element' algorithm>
Match a Selector Against a Pseudo-element</h3>

	This section defines how to <dfn export>match a selector against a pseudo-element</dfn>.

	APIs using this algorithm must provide a <var>selector</var>
	and a <var>pseudo-element</var>.
	They may optionally provide the same things they may optionally provide
	to the algorithm to <a>match a selector against an element</a>.

	This algorithm returns success or failure.

	For each <a>complex selector</a> in the given <var>selector</var>, if both:
	<ul>
		<li>the rightmost <a>simple selector</a> in the complex selector
		matches <var>pseudo-element</var>, and

		<li>the result of running <a>match a complex selector against an element</a>
		on the remainder of the <a>complex selector</a>
		(with just the rightmost simple selector
		of its rightmost complex selector removed),
		<var>pseudo-element</var>'s corresponding element,
		and any optional parameters provided to this algorithm
		returns success,
	</ul>
	then return success.

	Otherwise
	(that is, if this doesn't happen for any of the complex selectors in <var>selector</var>),
	return failure.

<h3 id='match-against-tree' algorithm>
Match a Selector Against a Tree</h3>

	This section defines how to <dfn export>match a selector against a tree</dfn>.

	APIs using this algorithm must provide a selector,
	and one or more <var>root elements</var>
	indicating the <a>trees</a> that will be searched by the selector.
	All of the <var>root elements</var> must share the same <a for=tree>root</a>,
	or else calling this algorithm is invalid.

	They may optionally provide:

	<ul>
		<li>
			A <a>scoping root</a>
			indicating the selector is <a>scoped</a>.
			If not specified,
			the <var>selector</var> defaults to being unscoped.

			Issue: This is now redundant with the <var>root elements</var>.

		<li>
			A set of <a>:scope elements</a>,
			which will match the '':scope'' pseudo-class.
			If not specified,
			then if the selector is a <a>scoped selector</a>,
			the set of <a>:scope elements</a> default to the <a>scoping root</a>;
			otherwise,
			it defaults to the <var>root elements</var>.

			Note: Note that if the selector is scoped,
			the scoping root is automatically taken as the <a>:scope element</a>,
			so it doesn't have to be provided explicitly
			unless a different result is desired.

		<li>
			Which <a>pseudo-elements</a> are allowed to show up in the match list.
			If not specified, this defaults to allowing all pseudo-elements.

			Issue: Only the ''::before'' and ''::after'' pseudo-elements are really
			handled in any way remotely like this.
	</ul>

	This algorithm returns a (possibly empty) list of elements.

	<ol>

		<li>
			Start with a list of <var>candidate elements</var>,
			which are the the <var>root elements</var>
			and all of their descendant elements,
			sorted in <a>shadow-including tree order</a>,
			unless otherwise specified.

		<li>
			If an optional scoping root was provided,
			then remove from the <var>candidate elements</var>
			any elements that are not
			<a>descendants</a> of the <a>scoping root</a>.

		<li>
			Initialize the <var>selector match list</var> to empty.

		<li>
			For each <var>element</var> in the set of
			<var>candidate elements</var>:

			<ol>
				<li>
				If the result of <a>match a selector against an element</a>
				for <var>element</var> and <var>selector</var> is success,
				add <var>element</var> to the <var>selector match list</var>.

				<li>
				For each possible pseudo-element associated with <var>element</var>
				that is one of the pseudo-elements allowed to show up in the match list,
				if the result of <a>match a selector against a pseudo-element</a>
				for the pseudo-element and <var>selector</var> is success,
				add the pseudo-element to the <var>selector match list</var>.

				Issue: The relative position of pseudo-elements
				in <var>selector match list</var> is undefined.
				There's not yet a context that exposes this information,
				but we need to decide on something eventually,
				before something <em>is</em> exposed.

	</ol>

<h2 id='dom-mapping'>
Appendix A: Guidance on Mapping Source Documents &amp; Data to an Element Tree</h2>

	<em>This section is informative.</em>

	The element tree structure described by the DOM is powerful and useful,
	but generic enough to model pretty much any language that describes tree-based data
	(or even graph-based, with a suitable interpretation).

	Some languages, like HTML, already have well-defined procedures
	for producing a DOM object from a resource.
	If a given language does not,
	such a procedure must be defined
	in order for Selectors to apply to documents in that language.

	At minimum,
	the document language must define what maps to the DOM concept of an "element".

	The primary one-to-many relationship between nodes--
	parent/child in tree-based structures,
	element/neighbors in graph-based structures--
	should be reflected as the child nodes of an element.

	Other features of the element should be mapped
	to something that serves a similar purpose to the same feature in DOM:

	<dl>
		<dt>type
		<dd>
			If the elements in the document language have some notion of "type"
			as a basic distinguisher between different groups of elements,
			it should be reflected as the "type" feature.

			If this "type" can be separated into a "basic" name
			and a "namespace" that groups names into higher-level groups,
			the latter should be reflected as the "namespace" feature.
			Otherwise, the element shouldn't have a "namespace" feature,
			and the entire name should be reflected as the "type" feature.

		<dt>id
		<dd>
			If some aspect of the element functions as a unique identifier across the document,
			it should be mapped to the "id" feature.

			Note: While HTML only allows an element to have a single ID,
			this should not be taken as a general restriction.
			The important quality of an ID is that each ID should be associated with a single element;
			a single element can validly have multiple IDs.

		<dt>classes and attributes
		<dd>
			Aspects of the element that are useful for identifying the element,
			but are not generally unique to elements within a document,
			should be mapped to the "class" or "attribute" features
			depending on if they're something equivalent to a "label" (a string by itself)
			or a "property" (a name/value pair)

		<dt>pseudo-classes and pseudo-attributes
		<dd>
			If any elements match any pseudo-classes or have any pseudo-elements,
			that must be explicitly defined.

			Issue: Some pseudo-classes are *syntactical*,
			like '':has()'' and '':matches()'',
			and thus should always work.
			Need to indicate that somewhere.
			Probably the structural pseudos always work
			whenever the child list is ordered.
	</dl>

	<div class='example'>
		For example, <a href="https://github.com/lloyd/JSONSelect">JSONSelect</a> is a library that uses selectors
		to extract information from JSON documents.

		<ul>
			<li>
				The "elements" of the JSON document
				are each array, object, boolean, string, number, or null.
				The array and object elements have their contents as children.

			<li>
				Each element's type is its JS type name:
				"array", "object", etc.

			<li>
				Children of an object
				have their key as a class.

			<li>
				Children of an array match the '':first-child'', '':nth-child()'', etc pseudo-classes.

			<li>
				The root object matches '':root''.

			<li>
				It additionally defines '':val()'' and '':contains()'' pseudo-classes,
				for matching boolean/number/string elements with a particular value
				or which contain a particular substring.
		</ul>

		This structure is sufficient to allow powerful, compact querying of JSON documents with selectors.
	</div>


<h2 id="changes">
Changes</h2>

	Significant changes since the <a href="https://www.w3.org/TR/2013/WD-selectors4-20130502/">2 May 2013 Working Draft</a> include:

	<ul>
		<li>Added '':is()'' pseudo-class.</li>
		<li>Split out <a>relative selectors</a> from <a>scoped selectors</a>,
			as these are different concepts that can be independently invoked.
		<li>Changed rules for absolutizing a relative selector.

		Issue: List changes + rationale?
		<li>Replaced subject indicator feature with '':has()''.
		<li>Changed the <css>:active-drop-target</css>, <css>:valid-drop-target</css>, <css>:invalid-drop-target</css> with '':drop()''.
		<li>Moved definition of <<An+B>> microsyntax to CSS Syntax.

		Issue: Semantic definition should probably move back here.
		<li>Added new sections:
			<ul>
				<li>[[#data-model]]

				Issue: Need to define tree for XML.
				<li>[[#api-hooks]]
					<ul>
						<li>Note that earlier versions of this section defined a section on <dfn id="evaluating-selectors">evaluating a selector</dfn>,
						but that section is no longer present.
						Specifications referencing that section should instead reference
						the algorithm to <a>match a selector against a tree</a>.
					</ul>
				</li>
			</ul>
		<li>Removed restriction on combinators within '':matches()'', '':not()'', '':nth-match()'', and '':nth-last-match()''.

			Issue: Do we have implementations of this? If not, maybe it's better to keep the restriction for level 4?
		<li>Defined <a>specificity</a> of a <a>selector list</a>. (Why?)
		<li>Removed the <css>:local-link</css> and reference combinator for lack of interest.
	</ul>


<h2 id="acknowledgements">
Acknowledgements</h2>

	The CSS working group would like to thank everyone who contributed
	to the <a href="https://www.w3.org/TR/css3-selectors">previous Selectors</a> specifications over the years,
	as those specifications formed the basis for this one.
	In particular, the working group would like to extend special thanks
	to the following for their specific contributions to Selectors Level 4:
	L. David Baron,
	Andrew Fedoniouk,
	Ian Hickson,
	Grey Hodge,
	Lachlan Hunt,
	Jason Cranford Teague

Privacy and Security Considerations {#priv-sec}
==============================================

This specification introduces no new privacy or security considerations,
as selectors do not provide any ability not already possible by walking the DOM manually.