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<h1>Selectors Level 4</h1>
<pre class='metadata'>
Group: CSSWG
Shortname: selectors
Level: 4
Status: ED
ED: http://dev.w3.org/csswg/selectors
TR: http://www.w3.org/TR/selectors/
Previous Version: http://www.w3.org/TR/2013/WD-selectors4-20130502/
Previous Version: http://www.w3.org/TR/2012/WD-selectors4-20120823/
Previous Version: http://www.w3.org/TR/2011/WD-selectors4-20110929/
Editor: Elika J. Etemad, Invited Expert, http://fantasai.inkedblade.net/contact
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
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: DocumentFragment, math, h1, shadow tree, box tree, query(), quirks mode, button, a, span, object, p, div, q, area, link, label, input, html, em, li, ol, pre
Link Defaults: dom-core-ls (interface) Document
</pre>
<style>
#selector-examples td:first-child {
white-space: nowrap;
}
</style>
<h2 id="context">
Introduction</h2>
<em>This section is not normative.</em>
A selector 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="http://en.wikipedia.org/wiki/Zen_Coding">Zen Coding</a>
</ul>
Selectors Levels 1, 2, and 3 are defined as the subsets of selector
functionality defined in the <a href="http://www.w3.org/TR/REC-CSS1">CSS1</a>,
<a href="http://www.w3.org/TR/CSS21/">CSS2.1</a>, and
<a href="http://www.w3.org/TR/css3-selectors/">Selectors Level 3</a>
specifications, respectively. This module defines Selectors Level 4.
<h3 id="placement">Module Interactions</h3>
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 "bar"
<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 othewise 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:drop</code>
<td>an E element that can possibly receieve 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
<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>[[#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>Fast</a> vs <a>Complete</a> Selector Profiles</h3>
Selectors are used in many different
A2B7
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 id='fast-profile'>fast</dfn>
<dd>
The <a>fast</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 id='complete-profile'>complete</dfn>
<dd>
The <a>complete</a> profile is appropriate for contexts which aren't extremely performance sensitive.
For example, the <a method for=Element>query()</a> method defined in [[DOM]] should use the ''complete'' profile.
It includes all of the selectors defined in this document.
</dl>
CSS implementations conformant to Selectors Level 4 must use the ''fast'' profile for CSS selection.
<p class='issue'>
The categorization of things into the "fast" or "complete" profiles needs implementor review.
If some things currently not in the fast 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 <dfn export>selector</dfn> can refer to a <a>simple selector</a>,
<a>compound selector</a>, <a>complex selector</a>, or <a>selector list</a>.
A <a>selector list</a> is a comma-separated list of <a>selectors</a>;
see <a href="#grouping">Selector Lists</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 id="compound" export>compound selector</dfn>
is a sequence of <a>simple selectors</a>
that are not separated by a <a>combinator</a>.
If it contains a <a>type selector</a> or <a>universal selector</a>,
that selector comes first in the sequence.
Only one type selector or universal selector is allowed in the sequence.
A <dfn id="simple" export>simple selector</dfn> represents an aspect of an element to be matched against.
A simple selector is either 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>.
A <dfn export>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:
whitespace,
“greater-than sign” (U+003E, <code>></code>),
“plus sign” (U+002B, <code>+</code>),
and “tilde” (U+007E, <code>~</code>).
Issue: Fill in more combinators.
An empty selector,
containing no <a>compound selector</a>,
is an <a href="#invalid">invalid selector</a>.
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>.
<h3 id='evaluating-selectors'>
Evaluating a Selector</h3>
This section describes how to <dfn export>evaluate a selector</dfn> against a set of elements.
APIs using this algorithm must provide a selector,
and one or more root elements
indicating the trees that will be searched by the selector.
They may optionally provide:
<ul>
<li>
a <a>scoping method</a> and <a>scoping root</a>.
If not specified, the selector defaults to being unscoped.
<li>
a set of <a>:scope elements</a>,
for resolving the '':scope'' pseudo-class against.
If not specified,
the set defaults to being empty.
If the selector is a <a>relative selector</a>,
the set of <a>:scope elements</a> must not be empty.
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 any.
If not specified, this defaults to allowing all pseudo-elements.
</ul>
A <a>selector</a> is evaluated against some initial list of elements:
the <dfn export>selector match list</dfn>.
The <a>selector match list</a> is initially populated with the root elements
provided to the algorithm,
and all elements reachable from them by traversing any number of child lists.
If the selector is <a>scope-contained</a>,
the <a>selector match list</a> is immediately filtered
to contain only elements that are either the <a>scoping root</a>
or descendants of the <a>scoping root</a>.
The selector is processed from left to right in order,
with <a>simple selectors</a> filtering the <a>selector match list</a>,
and <a>combinators</a> and <a>pseudo-elements</a> changing the <a>selector match list</a> into something new.
If the selector is <a>scope-contained</a>
then after each <a>combinator</a>
the <a>selector match list</a> must be filtered
to contain only elements that are either the <a>scoping root</a>
or descendants of the <a>scoping root</a>.
If the selector is <a>scope-filtered</a>,
then after the selector is finished processing,
the <a>selector match list</a> must be filtered
to contain only elements that are either the <a>scoping root</a>
or descendants of the <a>scoping root</a>.
After the selector is finished matching,
the <a>selector match list</a> must be filtered
to only contain elements and <a>pseudo-elements</a>
allowed by the invoker of this algorithm.
When this process is done, the elements in the <a>selector match list</a>
are the elements said to match the selector.
If the order of elements matter,
they must be sorted in document order,
unless otherwise specified.
<div class='example'>
For example, to evaluate the selector "div > i.name" against a document,
the <a>selector match list</a> is first set to all the elements in the entire document.
Then, the "div" type selector is evaluated, filtering the <a>selector match list</a>
to only contain elements with a tagname of "div".
Then, the ">" child combinator is evaluated, transforming the <a>selector match list</a>
by replacing each element currently in it with the element's children.
Then, the "i" type selector is evaluated, filtering the <a>selector match list</a>
to only contain elements with a tagname of "i".
Finally, the ".name" class selector is evaluated, filter the <a>selector match list</a>
to only contain elements with a class of "name".
</div>
Note: Many implementations of selectors instead evaluate them right to left,
as it's more efficient to do so in many cases.
This, as usual, is completely valid,
as long as it results in the same elements being returned
as the spec's algorithm would.
Issue: The relative position of pseudo-elements in the <a>selector match list</a> 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.
<h3 id='data-model'>
Data Model</h3>
Selectors are evaluated against a DOM tree. [[!DOM]]
Within this specification,
this may be referred to as the "document tree".
In addition to the information present in the document tree,
for the purpose of selectors,
elements are assumed to have arbitrary <a>pseudo-class</a> flags
and <a>pseudo-element</a> slots,
as defined by the given <a>pseudo-class</a> and <a>pseudo-element</a>.
<div class='example'>
For example, the first element child of each element
has a flag indicating that it matches the '':first-child'' pseudo-element.
For another example,
all elements have ''::before'' pseudo-elements,
but not only elements with <a>shadow trees</a> have ''::shadow'' pseudo-elements.
</div>
Five aspects of a DOM element are especially relevant for selectors:
<ul>
<li>The element's type (also known as tagname), which is a string.
<li>The element's namespace, which is a string.
<li>An ID, which is a string.
<li>Classes, which are strings.
<li>Attributes, which are pairs of strings consisting of an attribute name and an attribute value.
</ul>
These aspects are referred to as <dfn export lt="feature">features</dfn>.
The <a>simple selectors</a> which are keyed off of them
(<a>universal selector</a>, <a>type selectors</a>, <a>ID selectors</a>, <a>class selectors</a>, and <a>attribute selectors</a>)
are called <dfn export lt="feature selector">feature selectors</dfn>.
While elements may lack any of the individual <a>features</a>,
some elements are <dfn export>featureless</dfn>.
A <a>featureless</a> element does not match any <a>feature selector</a>,
or any selector that resolves based on <a>features</a>
(such as '':not(div)''),
with the exception of namespace selectors--
a <a>featureless</a> element is treated as having <em>all</em> namespaces.
Thus, only <a>pseudo-classes</a> or <a>pseudo-elements</a> can be used to select them.
Individual <a>featureless</a> elements may define additional restrictions on what kinds of selectors can match them.
<div class='example'>
For example, the <a>host element</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>
<details class='why'>
<summary>Why do featureless elements match all namespaces?</summary>
In a previous version of Selectors,
all <a>compound selectors</a> contained a <a>type selector</a>;
if you didn't supply one explicitly,
a <a>universal selector</a> was implied.
Default namespaces [[CSS3NAMESPACE]] affected the implied universal selector,
so that selectors like '':hover'' actually meant ''defaultNS|*:hover''.
The universal selector is no longer implied,
but default namespaces still exist,
and have to apply to all compound selectors.
The behavior of <a>featureless</a> elements,
like the <a>host element</a> in a shadow tree,
thus needs to accommodate that and work appropriately.
In this case, "appropriately" means "ignoring the default namespace".
</details>
When matching a selector against a document which is in <a>quirks mode</a>,
class and ID selectors must be matched <a lt="ASCII case-insensitive">ASCII case-insensitively</a>
against the classes and ID of the elements in the document.
<h4 id='dom-mapping'>
Guidance on Mapping Document Languages to DOM</h4>
The DOM document structure is powerful and useful,
but generic enough to model pretty much any langauge 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 reflected as <a>features</a> when possible,
mapping 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" <a>feature</a>.
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" <a>feature</a>.
Otherwise, the element shouldn't have a "namespace" <a>feature</a>,
and the entire name should be reflected as the "type" <a>feature</a>.
<dt>id
<dd>
If some aspect of the element functions as a unique identifier across the document,
it should be mapped to the "id" <a>feature</a>.
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" <a>features</a>
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>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>
<h3 id="scoping">
Scoped Selectors</h3>
Some host applications may choose to <dfn export lt="scope | scoped selector">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>).
There are two <dfn export lt="scoping method | selector scoping method">scoping methods</dfn> for selectors:
<dl>
<dt><dfn export lt="scope-contained | scope-contained selector">scope-contained</dfn>
<dd>
With this method of scoping, selectors match as if
the <a>scoping root</a> were the root of the document:
all <a>compound selectors</a> must be represented within the scope.
(The '':root'' pseudo-class, however,
still only matches the actual root of the document.)
<dt><dfn export lt="scope-filtered | scope-filtered selector">scope-filtered</dfn>
<dd>
With this method of scoping, a selector matches an element only if the element is within the scope,
even if other components of the selector are outside the scope.
(A <a>scoping element</a> is considered to be in scope.)
</dl>
<div class='example'>
For example,
the <code>element.querySelector()</code> function defined in [[DOM]]
allows the author to define a <a>scope-filtered</a> selector.
On the other hand, the selectors within an [[HTML5]] scoped stylesheet
define <a>scope-contained</a> selectors.
</div>
Note: If there are no <a>:scope elements</a> when a selector is evaluated,
the <a>scoping root</a> acts like 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>scope-contained</a> or <a>scope-filtered</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>
The <dfn export>pseudo-class</dfn> concept is introduced to 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.
The syntax of a <a>pseudo-class</a>
is a ":" (U+003A COLON)
followed by the name of the <a>pseudo-class</a>,
which is either an <a>identifier</a>,
or in the case of a <dfn export>functional pseudo-class</dfn>,
a function potentially containing arguments between parentheses
(similar to CSS functions).
No <a>white space</a> is allowed between the colon and the name of the <a>pseudo-class</a>,
and as usual for CSS syntax,
there are no spaces between the function name and its opening parenthesis.
As usual for CSS syntax,
<a>white space</a> is allowed arond the arguments between the parentheses
of a functional pseudo-class,
unless otherwise specified.
<a>Pseudo-classes</a> are identical to any other type of <a>simple selector</a>:
they're 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.
<a>Pseudo-class</a> names are <a>ASCII case-insensitive</a>.
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.
<a>Pseudo-classes</a> may be dynamic,
in the sense that an element can acquire or lose a pseudo-class
while a user interacts with the document.
<h3 id="pseudo-elements">Pseudo-elements</h3>
Similar to how <a>pseudo-classes</a> represent additional information
not directly present in the document tree
(or that is inconvenient to use),
a <dfn export>pseudo-element</dfn> is an <em>element</em>
not directly present in the document tree,
used to create abstractions about the document tree
beyond those specified by the document language.
<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.
<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>
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.
Due to legacy constraints,
user agents must allow ''::before'', ''::after'', ''::first-line'', and ''::first-letter'' pseudo-elements
to be written with a single colon rather than two,
similar to <a>pseudo-class</a> syntax.
Authors must always use the double-colon syntax for these <a>pseudo-elements</a>.
<a>Pseudo-elements</a> do not exist independently;
they belong to another element on the page,
called their <dfn export>originating element</dfn>.
When a <a>pseudo-element</a> is encountered in a selector as part of a <a>compound selector</a>,
the part of the selector to the left of the <a>pseudo-element</a> selects the <a>originating element</a> for the <a>pseudo-element</a>;
the part of the selector to the right, if any, applies to the <a>pseudo-element</a> itself.
If a <a>pseudo-element</a> <em>starts</em> a <a>compound selector</a>,
it's treated as if the selector to its left was 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.