csswg-drafts/selectors/Overview.src.html at e76de12cd354391517011884d3b56f2d54130e86 · xfq/csswg-drafts · GitHub

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html lang="en">
 <head>
  <meta charset=utf8>
  <title>Selectors Level 4</title>
  <link href="../default.css" type="text/css" rel="stylesheet">
  <style type="text/css">
    .tprofile td, th { vertical-align: baseline; padding: 0 0.5em; }
    .tprofile th { text-align: right; }
  </style>
  <link href="http://www.w3.org/StyleSheets/TR/W3C-WD.css" type="text/css" rel="stylesheet">
 </head>
 <body>
  <div class="head">
   <!--logo-->

   <h1 id="title">Selectors Level 4</h1>

   <h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>

   <dl>
    <dt>This version:
    <dd><a href="[VERSION]">
                 http://www.w3.org/TR/[YEAR]/ED-[SHORTNAME]-[CDATE]/</a>

    <dt>Editor's draft:
    <dd><a href="http://dev.w3.org/csswg/selectors4/">
                 http://dev.w3.org/csswg/selectors4/</a>

    <dt>Latest version of Selectors Level 4:
    <dd><a href="http://www.w3.org/TR/selectors4/">
                 http://
F328
www.w3.org/TR/selectors4/</a>

    <dt>Latest Selectors specification:
    <dd><a href="http://www.w3.org/TR/selectors/">
                 http://www.w3.org/TR/selectors/</a>

    <dt>Previous version:
    <dd><a href="http://www.w3.org/TR/2011/WD-selectors4-20110929/">
                 http://www.w3.org/TR/2011/WD-selectors4-20110929/</a>

  <dt>Feedback:
    <dd><a href="mailto:www-style@w3.org?subject=%5Bselectors4%5D%20feedback">www-style@w3.org</a>
        with subject line &ldquo;<kbd>[selectors4] <var>&hellip; message topic &hellip;</var></kbd>&rdquo;
        (<a rel="discussion" href="http://lists.w3.org/Archives/Public/www-style/">archives</a>)

    <dt>Editors:
    <dd class="vcard"><a class="url fn" href="http://fantasai.inkedblade.net/contact">Elika J. Etemad</a> (Mozilla)
    <dd class='vcard'>
      <a class='url fn' href="http://www.xanthir.com/contact/">Tab Atkins Jr.</a> (<span class='org'>Google</span>)

    <dt>Previous Editors:
    <dd class="vcard"><a lang="tr" class="url fn" href="http://www.tantek.com/">Tantek &Ccedil;elik</a> (Microsoft)
    <dd class="vcard"><span class="fn">Daniel Glazman</span> (Disruptive Innovations SARL)
    <dd class="vcard"><span class="fn">Ian Hickson</span> (Opera Softare ASA)
    <dd class="vcard"><span class="fn">Peter Linss</span> (Netscape/AOL)
    <dd class="vcard"><span class="fn">John Williams</span> (Quark, Inc.)
   </dl>

   <!--copyright-->

   <hr title="Separator for header">

  </div>

<h2 class="no-num no-toc" id="abstract">
Abstract</h2>

  <p><em>Selectors</em> are patterns that match against elements in a
  tree, and as such form one of several technologies that can be used
  to select nodes in an XML document. Selectors have been optimized for
  use with HTML and XML, and are designed to be usable in
  performance-critical code. They are a core component of
  <abbr title="Cascading Style Sheets">CSS</abbr> (Cascading
  Style Sheets), which uses Selectors to bind style properties to
  elements in the document.

  <p>Selectors Level 4 describes the selectors that already exist in
  [[!SELECT]], and further introduces new selectors for CSS and
  other languages that may need them.

  <h2 class="no-num no-toc" id="status">Status of this Document</h2>

  <!--status-->

  <p>This module is a somewhat-unstable Working Draft.
  <strong>If you are looking for a stable Selectors specification, use
  <a href="http://www.w3.org/TR/css3-selectors/">Selectors 3</a>.</strong>
  Read the <a href="http://www.w3.org/TR/CSS/">CSS Snapshot</a> for
  an overview of the CSS development process.
  See the <a href="#overview">Selectors Overview</a> for a summary of
  additions to level 3.

  <p>The following features are at-risk and may be dropped during the CR period
  if there is not sufficient implementer interest:
  the reference combinator,
  the column combinator,
  the '':invalid-drop'' and '':valid-drop'' pseudo-classes.

  <h2 class="no-num no-toc" id="contents">Table of Contents</h2>
  <!--toc-->

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

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

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

  <p>
    These expressions may be used for many things:

  <ul>
    <li>
      directly on an element to test whether it matches some criteria,
      such as in the <code>Element.matches()</code> function defined in [[SELECTORS-API2]]

    <li>
      applied to an entire tree of elements
      to filter it into a set of elements that match the criteria,
      such as in the <code>document.findAll()</code> function defined in [[SELECTORS-API2]]
      or the selector of a CSS style rule.

    <li>
      used "in reverse" to generate markup that would match a given selector,
      such as in <a href="http://haml.info/">HAML</a> or <a href="http://en.wikipedia.org/wiki/Zen_Coding">Zen Coding</a>
  </ul>


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

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

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

  <p>
    Pseudo-element selectors,
    which define abstract elements in a rendering tree,
    are not part of this specification:
    their generic syntax is described here,
    but, due to their close integration with the rendering model and irrelevance to other uses such as DOM queries,
    they will be defined in other modules.

<h2 id="overview">
Selectors Overview</h2>

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

  <p>A Selector represents a structure. This structure can be used as a
  condition (e.g. in a CSS rule) that determines which elements a
  selector matches in the document tree, or as a flat description of the
  HTML or XML fragment corresponding to that structure.

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

  <p>The following table summarizes the Selector syntax:

  <table class="data">
    <col class="pattern">
    <col class="meaning">
    <col class="section">
    <col class="level">
  <thead>
    <tr>
      <th>Pattern
      <th>Represents
      <th>Section
      <th>Level
  <tbody>
    <tr>
      <td><code>*</code>
      <td>any element
      <td><a href="#universal-selector">Universal selector</a>
      <td>2
    <tr>
      <td><code>E</code>
      <td>an element of type E
      <td><a href="#type-selectors">Type (tag name) selector</a>
      <td>1
  <tbody>
    <tr>
      <td><code>E:not(<var>s1</var>, <var>s2</var>)</code>
      <td>an E element that does not match either <i>compound selector</i> <var>s1</var>
          or <i>compound selector</i> <var>s2</var>
      <td><a href="#negation">Negation pseudo-class</a>
      <td>3/4
    <tr>
      <td><code>E:matches(<var>s1</var>, <var>s2</var>)</code>
      <td>an E element that matches <i>compound selector</i> <var>s1</var>
          and/or <i>compound selector</i> <var>s2</var>
      <td><a href="#matches">Matches-any pseudo-class</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E.warning</code>
      <td>an E element belonging to the class <code>warning</code>
          (the document language specifies how class is determined).
      <td><a href="#class-html">Class selectors</a>
      <td>1
    <tr>
      <td><code>E#myid</code>
      <td>an E element with ID equal to <code>myid</code>.
      <td><a href="#id-selectors">ID selectors</a>
      <td>1
    <tr>
      <td><code>E[foo]</code>
      <td>an E element with a <code>foo</code> attribute
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>2
    <tr>
      <td><code>E[foo="bar"]</code>
      <td>an E element whose <code>foo</code> attribute value is
          exactly equal to <code>bar</code>
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>2
    <tr>
      <td><code>E[foo="bar" i]</code>
      <td>an E element whose <code>foo</code> attribute value is
          exactly equal to any (ASCII-range) case-permutation of <code>bar</code>
      <td><a href="#attribute-case">Attribute selectors: Case-sensitivity</a>
      <td>4
    <tr>
      <td><code>E[foo~="bar"]</code>
      <td>an E element whose <code>foo</code> attribute value is
          a list of whitespace-separated values, one of which is
          exactly equal to <code>bar</code>
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>2
    <tr>
      <td><code>E[foo^="bar"]</code>
      <td>an E element whose <code>foo</code> attribute value
          begins exactly with the string "bar"
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>3
    <tr>
      <td><code>E[foo$="bar"]</code>
      <td>an E element whose <code>foo</code> attribute value
          ends exactly with the string <code>bar</code>
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>3
    <tr>
      <td><code>E[foo*="bar"]</code>
      <td>an E element whose <code>foo</code> attribute value
          contains the substring <code>bar</code>
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>3
    <tr>
      <td><code>E[foo|="en"]</code>
      <td>an E element whose <code>foo</code> attribute value is
          a hyphen-separated list of values beginning with <code>en</code>
      <td><a href="#attribute-selectors">Attribute selectors</a>
      <td>2
  <tbody>
    <tr>
      <td><code>E:dir(ltr)</code>
      <td>an element of type E in with left-to-right directionality
        (the document language specifies how directionality is determined)
      <td><a href="#dir-pseudo">The :dir() pseudo-class</a>
      <td>4
    <tr>
      <td><code>E:lang(zh, *-hant)</code>
      <td>an element of type E tagged as being either in Chinese
        (any dialect or writing system)
        or othewise written with traditional Chinese characters
      <td><a href="#lang-pseudo">The :lang() pseudo-class</a>
      <td>2/4
  <tbody>
    <tr>
      <td><code>E:any-link</code>
      <td>an E element being the source anchor of a hyperlink
      <td><a href="#any-link-pseudo">The hyperlink pseudo-class</a>
      <td>4
    <tr>
      <td><code>E:link</code>
          of which the target is not yet visited
      <td><a href="#link">The link history pseudo-classes</a>
      <td>1
    <tr>
      <td><code>E:visited</code>
      <td>an E element being the source anchor of a hyperlink
          of which the target is already visited
      <td><a href="#link">The link history pseudo-classes</a>
      <td>1
    <tr>
      <td><code>E:local-link</code>
      <td>an E element being the source anchor of a hyperlink
          of which the target is the current document
      <td><a href="#local-pseudo">The local link pseudo-class</a>
      <td>4
    <tr>
      <td><code>E:local-link(0)</code>
      <td>an E element being the source anchor of a hyperlink
          of which the target is within the current domain
      <td><a href="#local-pseudo">The local link pseudo-class</a>
      <td>4
    <tr>
      <td><code>E:target</code>
      <td>an E element being the target of the referring URL
      <td><a href="#target-pseudo">The target pseudo-class</a>
      <td>3
    <tr>
      <td><code>E:scope</code>
      <td>an E element being a designated reference element
      <td><a href="#scope-pseudo">The scope pseudo-class</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E:current</code>
      <td>an E element that is currently presented in a time-dimensional canvas
      <td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:current(<var>s</var>)</code>
      <td>an E element that is the deepest <code>:current</code> element that
          matches selector <var>s</var>
      <td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:past</code>
      <td>an E element that is in the past in a time-dimensional canvas
      <td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:future</code>
      <td>an E element that is in the future in a time-dimensional canvas
      <td><a href="#time-pseudos">Time-dimensional Pseudo-classes</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E:active</code>
      <td>an E element that is in an activated state
      <td><a href="#useraction-pseudos">The user action pseudo-classes</a>
      <td>1
    <tr>
      <td><code>E:hover</code>
      <td>an E element that is under the cursor,
          or that has a descendant under the cursor
      <td><a href="#useraction-pseudos">The user action pseudo-classes</a>
      <td>2
    <tr>
      <td><code>E:focus</code>
      <td>an E element that has user input focus
      <td><a href="#useraction-pseudos">The user action pseudo-classes</a>
      <td>2
    <tr>
      <td><code>E:active-drop</code>
      <td>an E element that will receive the item currently being dragged
      <td><a href="#drag-pseudos">The drag-and-drop pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:valid-drop</code>
      <td>an E element that could receive the item currently being dragged
      <td><a href="#drag-pseudos">The drag-and-drop pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:invalid-drop</code>
      <td>an E element that cannot receive the item currently being dragged, but could receive some other item
      <td><a href="#drag-pseudos">The drag-and-drop pseudo-classes</a>
      <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><a href="#enableddisabled">The :enabled and :disabled pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:read-only</code><br><code>E:read-write</code>
      <td>a user interface element E that
      <td><a href="#rw-pseudos">The mutability pseudo-classes</a>
      <td>3-UI/4
    <tr>
      <td><code>E:placeholder-shown</code>
      <td>an input control currently showing placeholder text
      <td><a href="#rw-pseudos">The placeholder-shown pseudo-class</a>
      <td>3-UI/4
    <tr>
      <td><code>E:default</code>
      <td>a user interface element E that
      <td><a href="#default-pseudo">The default option pseudo-class :default</a>
      <td>3-UI/4
    <tr>
      <td><code>E:checked</code>
      <td>a user interface element E that is checked/selected
         (for instance a radio-button or checkbox)
      <td><a href="#checked">The selected-option pseudo-class</a>
      <td>3
    <tr>
      <td><code>E:indeterminate</code>
      <td>a user interface element E that is in an indeterminate state
          (neither checked nor unchecked)
      <td><a href="#indeterminate">The indeterminate-value pseudo-class</a>
      <td>4
    <tr>
      <td><code>E:valid</code><br><code>E:invalid</code>
      <td>a user-input element E that
      <td><a href="#range-pseudos">The validity pseudo-classes</a>
      <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><a href="#range-pseudos">The range pseudo-classes</a>
      <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><a href="#opt-pseudos">The optionality pseudo-classes</a>
      <td>3-UI/4
    <tr>
      <td><code>E:required</code><br><code>E:optional</code>
      <td>a user-input element E with incorrect input
      <td><a href="#opt-pseudos">The optionality pseudo-classes</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E:root</code>
      <td>an E element, root of the document
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:empty</code>
      <td>an E element that has no children (not even text nodes)
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:blank</code>
      <td>an E element that has no content except maybe white space
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:nth-child(<var>n</var>)</code>
      <td>an E element, the <var>n</var>-th child of its parent
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:nth-last-child(<var>n</var>)</code>
      <td>an E element, the <var>n</var>-th child of its parent,
          counting from the last one
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:first-child</code>
      <td>an E element, first child of its parent
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>2
    <tr>
      <td><code>E:last-child</code>
      <td>an E element, last child of its parent
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:only-child</code>
      <td>an E element, only child of its parent
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:nth-of-type(<var>n</var>)</code>
      <td>an E element, the <var>n</var>-th sibling of its type
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:nth-last-of-type(<var>n</var>)</code>
      <td>an E element, the <var>n</var>-th sibling of its type,
          counting from the last one
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:first-of-type</code>
      <td>an E element, first sibling of its type
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:last-of-type</code>
      <td>an E element, last sibling of its type
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:only-of-type</code>
      <td>an E element, only sibling of its type
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>3
    <tr>
      <td><code>E:nth-match(<var>n</var> of <var title=''>selector</var>)</code>
      <td>an E element, the <var>n</var>-th sibling matching <var title=''>selector</var>
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:nth-last-match(<var>n</var> of <var title=''>selector</var>)</code>
      <td>an E element, the <var>n</var>-th sibling matching <var title=''>selector</var>,
          counting from the last one
      <td><a href="#structural-pseudos">Structural pseudo-classes</a>
      <td>4
  <tbody>
    <tr>
      <td><code>E F</code>
      <td>an F element descendant of an E element
      <td><a href="#descendant-combinators">Descendant combinator</a>
      <td>1
    <tr>
      <td><code>E > F</code>
      <td>an F element child of an E element
      <td><a href="#child-combinators">Child combinator</a>
      <td>2
    <tr>
      <td><code>E + F</code>
      <td>an F element immediately preceded by an E element
      <td><a href="#adjacent-sibling-combinators">Next-sibling combinator</a>
      <td>2
    <tr>
      <td><code>E ~ F</code>
      <td>an F element preceded by an E element
      <td><a href="#general-sibling-combinators">Following-sibling combinator</a>
      <td>3
    <tr>
      <td><code>E /foo/ F</code>
      <td>an F element ID-referenced by an E element's <code>foo</code> attribute
      <td><a href="#idref-combinators">Reference combinator</a>
      <td>4
    <tr>
      <td><code>E! > F</code>
      <td>an E element parent of an F element
      <td><a href="#subject">Determining the subject of a selector</a> +
          <a href="#child-combinators">Child combinator</a>
      <td>4
  <tbody>
    <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><a href="#table-pseudos">Grid-Structural pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:nth-column(<var>n</var>)</code>
      <td>an E element that represents a cell belonging to the
          <var>n</var>th column in a grid/table
      <td><a href="#table-pseudos">Grid-Structural pseudo-classes</a>
      <td>4
    <tr>
      <td><code>E:nth-last-column(<var>n</var>)</code>
      <td>an E element that represents a cell belonging to the
          <var>n</var>th column in a grid/table, counting from the last one
      <td><a href="#table-pseudos">Grid-Structural pseudo-classes</a>
      <td>4
  </tbody></table>

  <p class="note">Some Level 4 selectors (noted above as "3-UI") were
    introduced in [[CSS3UI]].

<h3 id="profiles">
<i>Fast</i> vs <i>Complete</i> Selector Profiles</h3>

  <p>
    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 id='fast-profile'>fast</dfn>
    <dd>
      The <i>fast</i> 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><i>Combinators</i> within <code>:matches()</code>, <code>:not()</code>, <code>:nth-match()</code>, and <code>:nth-last-match()</code>.
          <p class='issue'>We are considering whether this restriction should be lifted.
        <li>The <a href="#idref-combinators">reference combinator</a>
        <li>The <a href="#subject">subject indicator</a>
      </ul>

    <dt><dfn id='complete-profile'>complete</dfn>
    <dd>
      The <i>complete</i> profile is appropriate for contexts which aren't extremely performance sensitive.
      For example, implementations of the Selectors API specification [[SELECTORS-API]] should use the ''complete'' profile.
      It includes all of the selectors defined in this document.
  </dl>

  <p>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>

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

  <p>A <i>selector list</i> is a comma-separated list of <i>selectors</i>;
    see <a href="#grouping">Selector Lists</a>.

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

  <p>A <dfn id="compound">compound selector</dfn>
  is a chain of <i>simple selectors</i>
  that are not separated by a <i>combinator</i>. It
  always begins with a <i>type selector</i> or a
  (possibly implied) <i>universal selector</i>.
  No other type selector or universal selector is allowed in the sequence.

  <p>
    A <dfn id="simple">simple selector</dfn> is either a
    <i>type selector</i>,
    <i>universal selector</i>,
    <i>attribute selector</i>,
    <i>class selector</i>,
    <i>ID selector</i>,
    or <i>pseudo-class</i>.

  <p>A <dfn>combinator</dfn> is punctuation that represents a particular
  kind of relationship between the <i>compound selectors</i> on either side.
  Combinators in Selectors level 4 include: whitespace, &quot;greater-than
  sign&quot; (U+003E, <code>></code>), &quot;plus sign&quot; (U+002B,
  <code>+</code>) and &quot;tilde&quot; (U+007E, <code>~</code>).
  <a href="#whitespace">White space</a> may appear between a combinator
  and the simple selectors around it.

  <p>An empty selector, containing no <i>compound selector</i>, is an
  <a href="#invalid">invalid selector</a>.

<h3 id="subject">
Determining the Subject of a Selector</h3>

  <p>The elements of a document tree that are represented by a selector
  are the <dfn title="subject">subjects</dfn> of the selector.

  <p>By default, the subjects of a selector are the elements represented
  by the last <i>compound selector</i> in the selector.
  Thus a selector consisting of a single <i>compound selector</i>
  represents any element satisfying its requirements
  Prepending another <i>compound selector</i> and a combinator
  to a sequence imposes additional matching constraints,
  so the subjects of the selector are always a subset of
  the elements represented by the last <i>compound selector</i>.

  <p>As a feature of the <i>complete</i> Selectors profile,
  the subject of the selector can be explicitly identified
  by prepending an exclamation mark (!)
  to one of the <i>compound selectors</i> in a selector.
  Although the element structure that the selector represents is the same
  with or without the exclamation mark, indicating the subject in this way
  can change which <i>compound selector</i> represents the subject in that structure.

  <p class="issue">
    Should the exclamation mark be prepended or appended to the subject? Or both?
    Or prepend two, to avoid the "! = not" issue?

  <div class="example">
  <p>For example, the following selector represents a list item <code>LI</code>
    unique child of an ordered list <code>OL</code>:
    <pre>OL > LI:only-child</pre>
    <p>However the following one represents an ordered list <code>OL</code>
    having a unique child, that child being a <code>LI</code>:
    <pre>!OL > LI:only-child</pre>
    <p>The tree structures represented by these two selectors are the same,
    but the subjects of the selectors are not.
  </div>

  <p>If multiple subject indicators are present in a selector, the subjects
  of the selectors are the elements represented by all <i>compound selectors</i>
  prepended by a subject indicator.</p>

  <div class="example">
    <p>For example, the following selectors represents all paragraphs of class
      <code>warning</code> contained in a div AND their containing <code>div</code> elements:</p>
    <pre>!div > !p.warning</pre>
  </div>

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

  <p>Some host applications may choose to <dfn title="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>scoping root</dfn>,
  and may be either a true element (the <dfn>scoping element</dfn>)
  or a <dfn title="virtual scoping root">virtual</dfn> one (such as a <code>DocumentFragment</code>).

  <p>There are two methods of scoping selectors:

  <dl>
    <dt><dfn title="scope-contained | scope-contained selector">scope-contained selectors</dfn>
    <dd>
      With this method of scoping, selectors match as if
      the <i>scoping root</i> were the root of the document:
      all <i>compound selectors</i> must be represented within the scope.
      (The <code>:root</code> pseudo-class, however,
      still only matches the actual root of the document.)

    <dt><dfn title="scope-filtered | scope-filtered selector">scope-filtered</dfn> selectors
    <dd>
      With this method of scoping, a selector matches if
      the <i>subject</i> of the selector is within the scope,
      even if other components of the selector are outside the scope.
      (A <i>scoping element</i> is considered to be in scope.)
  </dl>

  <div class='example'>
    <p>
      For example,
      the <code>element.querySelector()</code> function defined in [[SELECTORS-API2]]
      allows the author to define a <i>scope-filtered</i> selector.

    <p>
      On the other hand, the selectors within an [[HTML5]] scoped stylesheet
      define <i>scope-contained</i> selectors.
  </div>

  <p>Unless otherwise specified,
  the <i>scoping root</i> also forms the <i>reference element set</i>
  and matches the <code>:scope</code> pseudo-class.

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

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

  <p>
    Relative selectors, once absolutized, can additionally be <i>scope-contained</i>
    (such as within the <code>::distributed()</code> pseudo-class [[SHADOWDOM]])
    or <i>scope-filtered</i>.

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

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

  <p>
    If the <i>reference element set</i> is empty or consists of one or more elements:

  <ol>
    <li>
      If the selector starts with a <i>combinator</i> other than the <i>descendant combinator</i>,
      prepend <code>:scope</code> as the initial <i>compound selector</i>.

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

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

  <p class='issue'>
    The above algorithm is different from Selectors API: its step 2, which bailed if the reference element set was empty, has been removed.
    This is because handling an explicitly empty list should return nothing;
    if a reference element set was not given at all,
    Selectors API 2 should be amended to say that the selector must be absolute already.
    (Broken example: doing <code>document.find('img', [list-of-links]))</code> finds images that are descendants of links normally,
    but instead finds all images in the document if the list-of-links is empty.)

  <p>
    Otherwise, if the <i>reference element set</i> consists of only a <i>virtual scoping root</i>:

  <ol>
    <li>
      If the selector starts with a <i>child combinator</i>,
      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 <i>descendant combinator</i>,
      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>

  <p>
    Otherwise, this is a spec error.  Please report it to the relevant standards body.

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


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

  <p>The <dfn>pseudo-class</dfn> concept is introduced to permit selection based on
  information that lies outside of the document tree
  or that can be awkward or impossible to express using the other simple selectors.

  <p>A <i>pseudo-class</i> always consists of a “colon”
  (<code>:</code>) followed by the name of the <i>pseudo-class</i> and, for
  <dfn>functional pseudo-classes</dfn>, by one or more arguments between parentheses
  (similar to CSS functions).
  White space is optionally allowed between the parentheses and the argument,
  but not between the pseudo-class name and the parentheses.
  If arguments are separated by commas, white space is optionally allowed
  before/after each comma.

  <p><i>Pseudo-classes</i> are allowed in all <i>compound selectors</i> contained in a selector.
  <i>Pseudo-classes</i> are allowed anywhere in a <i>compound selector</i>
  after the leading type selector or (possibly omitted) universal selector.
  <i>Pseudo-class</i> names are <a href="#case-sensitive">ASCII case-insensitive</a>.
  Some <i>pseudo-classes</i> are mutually exclusive
  (such that a <i>compound selector</i> containing them, while valid, will never match anything),
  while others can apply simultaneously to the same element.
  <i>Pseudo-classes</i> may be dynamic,
  in the sense that an element can acquire or lose a pseudo-class
  while a user interacts with the document.

  <p>Dynamic <i>pseudo-classes</i> classify elements on characteristics
  other than their name, attributes, or content,
  but rather on characteristics that cannot be deduced from the document tree.
  They do not appear in or modify the document source or document tree.

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

  <p><dfn id='pseudo-element' title="pseudo-element">Pseudo-elements</dfn> create abstractions about the document tree
  beyond those specified by the document language.
  For instance, document languages do not offer mechanisms to access
  the first letter or first line of an element's content.
  <i>Pseudo-elements</i> allow authors to refer
  to this otherwise inaccessible information.
  <i>Pseudo-elements</i> may also provide authors a way
  to refer to content that does not exist in the source document
  (e.g., the <code>::before</code> and <code>::after</code> <i>pseudo-elements</i>
  give access to generated content in CSS [[CSS21]]).

  <p>A <i>pseudo-element</i> is made of two colons (<code>::</code>)
  followed by the name of the <i>pseudo-element</i>.
  <i>Pseudo-element</i> names are <a href="#case-sensitive">ASCII case-insensitive</a>.

  <p>This <code>::</code> notation was chosen in order to establish
  a discrimination between <i>pseudo-classes</i> (which subclass existing elements)
  and <i>pseudo-elements</i> (which are elements not represented in the document tree).
  However, for compatibility with existing style sheets,
  user agents must also accept the previous one-colon notation
  for <i>pseudo-elements</i> introduced in CSS levels 1 and 2
  (namely, <code>:first-line</code>, <code>:first-letter</code>,
  <code>:before</code> and <code>:after</code>).
  This compatibility notation is not allowed any other <i>pseudo-elements</i>.

  <p class="note">A future version of this specification may allow
  multiple pseudo-elements per selector.

  <p>Syntactically, a <i>pseudo-element</i> immediately follows
  the <i>compound selector</i> representing its <dfn>originating element</dfn>,
  i.e. the element to which it is associated.
  Unless otherwise overridden by the definition of the <i>pseudo-element</i>:
  <ul>
    <li>only one <i>pseudo-element</i> may appear per <i>complex selector</i>
    <li>the <i>pseudo-element</i> must appear after the <i>compound selector</i>
      that represents the <a href="#subject">subjects</a> of the selector
    <li>the <i>pseudo-element</i> may appear only if the subject of the selector
      is the last <i>compound selector</i> in the selector.
  </ul>

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

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

  <p>The host language defines which pseudo-elements exist and their meaning.
  For CSS, [[CSS21]] defines the ::before, ::after, ::first-line and ::first-letter pseudo-elements.

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

  <p>All Selectors syntax is case-insensitive within the ASCII range
  (i.e. [a-z]  and [A-Z] are equivalent),
  except for 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.
  For example,
  <a href="http://www.whatwg.org/html/selectors.html#case-sensitivity">in HTML, element names are case-insensitive</a>,
  but in XML, they are case-sensitive.
  Case sensitivity of namespace prefixes is defined in [[!CSS3NAMESPACE]].
  Case sensitivity of <i>language ranges</i> is defined in the <code>:lang()</code> section.

  <p><dfn id="whitespace">White space</dfn> in Selectors consists of the
  characters SPACE (U+0020), TAB (U+0009), LINE FEED (U+000A),
  CARRIAGE RETURN (U+000D), and FORM FEED (U+000C) can occur in whitespace.
  Other space-like characters, such as EM SPACE (U+2003) and
  IDEOGRAPHIC SPACE (U+3000), are never part of white space.

  <p>Characters in Selectors can be escaped with a backslash
  according to the same <a href="http://www.w3.org/TR/CSS21/syndata.html#characters">escaping rules</a> as CSS. [[!CSS21]]
  Note that escaping a character "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">
Namespaces</h3>

  <p>Certain selectors support namespace prefixes. The
  mechanism by which namespace prefixes are <dfn id="nsdecl">declared</dfn> should
  be specified by the language that uses Selectors. If the language does
  not specify a namespace prefix declaration mechanism, then no prefixes
  are declared. In CSS, namespace prefixes are declared with the
  <a href="http://www.w3.org/TR/css3-namespace/#declaration"><code>@namespace</code></a>
  rule. [[!CSS3NAMESPACE]]

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

  <p>User agents must observe the rules for handling <dfn title="invalid selector|invalid">invalid selectors</dfn>:
  <ul>
    <li>a parsing error in a selector,
      e.g. an unrecognized token or a token which is not allowed at the current parsing point,
      causes that selector to be invalid
    <li>a simple selector containing an <a href="#namespaces">undeclared namespace prefix</a> is invalid
    <li>a selector containing an invalid simple selector, an invalid combinator
      or an invalid token is invalid.
    <li>a selector list containing an invalid selector is invalid.
  </ul>

  <p>An <i>invalid selector</i> represents nothing.


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

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

  <p>A comma-separated list of selectors represents the union of all
  elements selected by each of the individual selectors in the
  <dfn>selector list</dfn>.
  (A comma is U+002C.) For example, in CSS when several selectors share
  the same declarations, they may be grouped into a comma-separated
  list. White space may appear before and/or after the comma.

  <div class="example">
    <p>CSS example:
    <p>In this example, we condense three rules with identical
    declarations into one. Thus,
    <pre><!--
    -->h1 { font-family: sans-serif }&#xa;<!--
    -->h2 { font-family: sans-serif }&#xa;<!--
    -->h3 { font-family: sans-serif }</pre>
    <p>is equivalent to:
    <pre>h1, h2, h3 { font-family: sans-serif }</pre>
  </div>

  <p><strong>Warning</strong>: the equivalence is true in this example
  because all the selectors are valid selectors. If just one of these
  selectors were invalid, the entire <i>selector list</i> would be
  invalid. This would invalidate the rule for all three heading
  elements, whereas in the former case only one of the three individual
  heading rules would be invalidated.

  <div class="example">
    <p>Invalid CSS example: