csswg-drafts/css-transitions/Overview.html at c903d5cad78b505c0700f19a0bb47d63366ca732 · w3c/csswg-drafts · GitHub

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<!doctype html>
<html lang="en">
 <head>

  <meta content="text/html; charset=utf-8" http-equiv="Content-Type">


  <title>CSS Transitions</title>


  <link href="../default.css" rel="stylesheet" type="text/css">


  <style>
  body {
    background: url("https://www.w3.org/StyleSheets/TR/logo-ED") top left no-repeat white;
    background-attachment: fixed;
    color: black;
    font-family: sans-serif;
    margin: 0 auto;
    max-width: 50em;
    padding: 2em 1em 2em 70px;
  }
  :link { color: #00C; background: transparent }
  :visited { color: #609; background: transparent }
  a[href]:active { color: #C00; background: transparent }
  a[href]:hover { background: #ffa }

  a[href] img { border-style: none }

  h1, 
F889
h2, h3, h4, h5, h6 { text-align: left }
  h1, h2, h3 { color: #005A9C; }
  h1 { font: 170% sans-serif }
  h2 { font: 140% sans-serif }
  h3 { font: 120% sans-serif }
  h4 { font: bold 100% sans-serif }
  h5 { font: italic 100% sans-serif }
  h6 { font: small-caps 100% sans-serif }

  .hide { display: none }

  div.head { margin-bottom: 1em }
  div.head h1 { margin-top: 2em; clear: both }
  div.head table { margin-left: 2em; margin-top: 2em }

  p.copyright { font-size: small }
  p.copyright small { font-size: small }

  pre { margin-left: 2em }
  dt { font-weight: bold }

  ul.toc, ol.toc {
    list-style: none;
  }
  </style>


  <meta content="Bikeshed 1.0.0" name="generator">


  <script defer="" src="//test.csswg.org/harness/annotate.js#css-transitions-1_dev/css-transitions-1" type="text/javascript"></script>


  <style type="text/css">
    table.animatable-properties {
      border-collapse: collapse;
    }
    table.animatable-properties td {
      padding: 0.2em 1em;
      border: 1px solid black;
    }
    div.prod { margin: 1em 2em; }
  </style>


 </head>


 <body class="h-entry">

  <div class="head">

   <p data-fill-with="logo"><a class="logo" href="http://www.w3.org/">
    <img alt="W3C" height="48" src="https://www.w3.org/Icons/w3c_home" width="72">
</a>
</p>

   <h1 class="p-name no-ref" id="title">CSS Transitions</h1>

   <h2 class="no-num no-toc no-ref heading settled" id="subtitle"><span class="content">Editor’s Draft,
    <time class="dt-updated" datetime="2015-02-19">19 February 2015</time></span></h2>

   <div data-fill-with="spec-metadata">
    <dl>
     <dt>This version:
     <dd><a class="u-url" href="http://dev.w3.org/csswg/css-transitions/">http://dev.w3.org/csswg/css-transitions/</a>
     <dt>Latest version:
     <dd><a href="http://www.w3.org/TR/css3-transitions/">http://www.w3.org/TR/css3-transitions/</a>
     <dt>Previous Versions:
     <dd><a href="http://www.w3.org/TR/2013/WD-css3-transitions-20131119/" rel="previous">http://www.w3.org/TR/2013/WD-css3-transitions-20131119/</a>
     <dt>Feedback:
     <dd><span><a href="mailto:www-style@w3.org?subject=%5Bcss-transitions%5D%20feedback">www-style@w3.org</a> with subject line “<kbd>[css-transitions] <var>… message topic …</var></kbd>” (<a href="http://lists.w3.org/Archives/Public/www-style/" rel="discussion">archives</a>)</span>
     <dt>Test Suite:
     <dd><a href="http://test.csswg.org/suites/css-transitions-1_dev/nightly-unstable/">http://test.csswg.org/suites/css-transitions-1_dev/nightly-unstable/</a>
     <dt>Issue Tracking:
     <dd><a href="https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&amp;amp;product=CSS&amp;amp;component=Transitions&amp;amp;resolution=---&amp;amp;status_whiteboard=defer%20to%20level%202&amp;amp;status_whiteboard_type=notregexp">Bugzilla bugs for this level</a>
     <dd><a href="https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&amp;amp;product=CSS&amp;amp;component=Transitions&amp;amp;resolution=---">Bugzilla bugs for all levels</a>
     <dt class="editor">Editors:
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-url url" href="http://dbaron.org/">L. David Baron</a> (<span class="p-org org">Mozilla</span>)
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:dino@apple.com">Dean Jackson</a> (<span class="p-org org">Apple Inc</span>)
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:hyatt@apple.com">David Hyatt</a> (<span class="p-org org">Apple Inc</span>)
     <dd class="editor p-author h-card vcard"><a class="p-name fn u-email email" href="mailto:cmarrin@apple.com">Chris Marrin</a> (<span class="p-org org">Apple Inc</span>)
    </dl>
   </div>

   <div data-fill-with="warning"></div>

   <p class="copyright" data-fill-with="copyright"><a href="http://www.w3.org/Consortium/Legal/ipr-notice#Copyright">Copyright</a> © 2015 <a href="http://www.w3.org/"><abbr title="World Wide Web Consortium">W3C</abbr></a><sup>®</sup> (<a href="http://www.csail.mit.edu/"><abbr title="Massachusetts Institute of Technology">MIT</abbr></a>, <a href="http://www.ercim.eu/"><abbr title="European Research Consortium for Informatics and Mathematics">ERCIM</abbr></a>, <a href="http://www.keio.ac.jp/">Keio</a>, <a href="http://ev.buaa.edu.cn/">Beihang</a>). W3C <a href="http://www.w3.org/Consortium/Legal/ipr-notice#Legal_Disclaimer">liability</a>, <a href="http://www.w3.org/Consortium/Legal/ipr-notice#W3C_Trademarks">trademark</a> and <a href="http://www.w3.org/Consortium/Legal/copyright-documents">document use</a> rules apply.
</p>

   <hr title="Separator for header">
</div>


  <h2 class="no-num no-toc no-ref heading settled" id="abstract"><span class="content">Abstract</span></h2>

  <div class="p-summary" data-fill-with="abstract">
   <p>CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.</p>

<a href="http://www.w3.org/TR/CSS/">CSS</a> is a language for describing the rendering of structured documents
(such as HTML and XML)
on screen, on paper, in speech, etc.</div>


  <h2 class="no-num no-toc no-ref heading settled" id="status"><span class="content">Status of this document</span></h2>

  <div data-fill-with="status">
   <p>
	This is a public copy of the editors’ draft.
	It is provided for discussion only and may change at any moment.
	Its publication here does not imply endorsement of its contents by W3C.
	Don’t cite this document other than as work in progress.

</p>
   <p>
	The (<a href="http://lists.w3.org/Archives/Public/www-style/">archived</a>) public mailing list
	<a href="mailto:www-style@w3.org?Subject=%5Bcss-transitions%5D%20PUT%20SUBJECT%20HERE">www-style@w3.org</a>
	(see <a href="http://www.w3.org/Mail/Request">instructions</a>)
	is preferred for discussion of this specification.
	When sending e-mail,
	please put the text “css-transitions” in the subject,
	preferably like this:
	“[css-transitions] <em>…summary of comment…</em>”

</p>
   <p>
	This document was produced by the <a href="http://www.w3.org/Style/CSS/members">CSS Working Group</a>
	(part of the <a href="http://www.w3.org/Style/">Style Activity</a>).

</p>
   <p>
	This document was produced by a group operating under
	the <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/">5 February 2004 W3C Patent Policy</a>.
	W3C maintains a <a href="http://www.w3.org/2004/01/pp-impl/32061/status" rel="disclosure">public list of any patent disclosures</a>
	made in connection with the deliverables of the group;
	that page also includes instructions for disclosing a patent.
	An individual who has actual knowledge of a patent which the individual believes contains <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#def-essential">Essential Claim(s)</a>
	must disclose the information in accordance with <a href="http://www.w3.org/Consortium/Patent-Policy-20040205/#sec-Disclosure">section 6 of the W3C Patent Policy</a>.

</p>
   <p>
	This document is governed by the <a href="http://www.w3.org/2014/Process-20140801/" id="w3c_process_revision">1 August 2014 W3C Process Document</a>.

</p>
   <p>
	<strong>This document</strong> is expected to be relatively close to last call.  While some issues raised have yet to be addressed, new features are extremely unlikely to be considered for this level.
</p></div>

  <div data-fill-with="at-risk"></div>


  <h2 class="no-num no-toc no-ref heading settled" id="contents"><span class="content">Table of Contents</span></h2>

  <div data-fill-with="table-of-contents" role="navigation">
   <ul class="toc" role="directory">
    <li><a href="#introduction"><span class="secno">1</span> <span class="content">Introduction</span></a>
    <li><a href="#transitions"><span class="secno">2</span> <span class="content"><span>Transitions</span></span></a>
     <ul class="toc">
      <li><a href="#transition-property-property"><span class="secno">2.1</span> <span class="content"><span>
        The <span class="property" data-lt="transition-property">transition-property</span> Property
      </span></span></a>
      <li><a href="#transition-duration-property"><span class="secno">2.2</span> <span class="content"><span>
        The <span class="property" data-lt="transition-duration">transition-duration</span> Property
      </span></span></a>
      <li><a href="#transition-timing-function-property"><span class="secno">2.3</span> <span class="content"><span>
        The <span class="property" data-lt="transition-timing-function">transition-timing-function</span> Property
      </span></span></a>
      <li><a href="#transition-delay-property"><span class="secno">2.4</span> <span class="content"><span>
        The <span class="property" data-lt="transition-delay">transition-delay</span> Property
      </span></span></a>
      <li><a href="#transition-shorthand-property"><span class="secno">2.5</span> <span class="content"><span>
        The <span class="property" data-lt="transition">transition</span> Shorthand Property
      </span></span></a>
     </ul>
    <li><a href="#starting"><span class="secno">3</span> <span class="content">
        Starting of transitions
      </span></a>
     <ul class="toc">
      <li><a href="#reversing"><span class="secno">3.1</span> <span class="content">
        Faster reversing of interrupted transitions
      </span></a>
     </ul>
    <li><a href="#application"><span class="secno">4</span> <span class="content">
        Application of transitions
      </span></a>
    <li><a href="#complete"><span class="secno">5</span> <span class="content">Completion of transitions</span></a>
    <li><a href="#transition-events"><span class="secno">6</span> <span class="content"><span>
        Transition Events
      </span></span></a>
    <li><a href="#animatable-types"><span class="secno">7</span> <span class="content"><span>
        Animation of property types
      </span></span></a>
    <li><a href="#animatable-properties"><span class="secno">8</span> <span class="content"><span>
        Animatable properties
      </span></span></a>
     <ul class="toc">
      <li><a href="#animatable-css"><span class="secno">8.1</span> <span class="content"><span>
        Properties from CSS
      </span></span></a>
      <li><a href="#animatable-svg"><span class="secno">8.2</span> <span class="content"><span>
       Properties from SVG
     </span></span></a>
     </ul>
    <li><a href="#changes"><span class="secno">9</span> <span class="content">Changes since Working Draft of 19 November 2013</span></a>
    <li><a href="#acknowledgments"><span class="secno">10</span> <span class="content">Acknowledgments</span></a>
    <li><a href="#conformance"><span class="secno"></span> <span class="content">
Conformance</span></a>
     <ul class="toc">
      <li><a href="#conventions"><span class="secno"></span> <span class="content">
Document conventions</span></a>
      <li><a href="#conformance-classes"><span class="secno"></span> <span class="content">
Conformance classes</span></a>
Partial implementations</span></a>
      <li><a href="#experimental"><span class="secno"></span> <span class="content">
Experimental implementations</span></a>
      <li><a href="#testing"><span class="secno"></span> <span class="content">
Non-experimental implementations</span></a>
     </ul>
    <li><a href="#references"><span class="secno"></span> <span class="content">References</span></a>
     <ul class="toc">
      <li><a href="#normative"><span class="secno"></span> <span class="content">Normative References</span></a>
      <li><a href="#informative"><span class="secno"></span> <span class="content">Informative References</span></a>
     </ul>
    <li><a href="#index"><span class="secno"></span> <span class="content">Index</span></a>
    <li><a href="#property-index"><span class="secno"></span> <span class="content">Property Index</span></a>
    <li><a href="#idl-index"><span class="secno"></span&g
8D73
t; <span class="content">IDL Index</span></a>
   </ul></div>

  <main>


   <h2 class="heading settled" data-level="1" id="introduction"><span class="secno">1. </span><span class="content">Introduction</span><a class="self-link" href="#introduction"></a></h2>


   <p><em>This section is not normative.</em>
      </p>
   <p>
        This document introduces new CSS features to enable <em>implicit transitions</em>, which describe how CSS properties can be made to change smoothly from one value to another over a given duration.
      </p>


   <h2 class="heading settled" data-level="2" id="transitions"><span class="secno">2. </span><span class="content"><span id="transitions-">Transitions</span></span><a class="self-link" href="#transitions"></a></h2>

   <p>
        Normally when the value of a CSS property changes, the rendered result is instantly updated, with the affected elements immediately changing from the old property value to the new property value. This section describes a way to specify transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.
      </p>

   <p>
        For example, suppose that transitions of one second have been defined on the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css2/visuren.html#propdef-left">left</a> and
        <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css2/colors.html#propdef-background-color">background-color</a> properties. The following diagram illustrates the effect of updating those properties on an element, in this case moving it to the right and changing the background from red to blue. This assumes other transition parameters still have their default values.
      </p>

   <div class="figure">
        <img alt="" src="transition1.png">
      </div>

   <p class="caption">
        Transitions of <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css2/visuren.html#propdef-left">left</a> and <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css2/colors.html#propdef-background-color">background-color</a>
      </p>

   <p>
        Transitions are a presentational effect. The <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-cascade-3/#computed-value">computed value</a> of a property transitions over time from the old value to the new value. Therefore if a script queries the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-cascade-3/#computed-value">computed value</a> of a property (or other data depending on it) as it is transitioning, it will see an intermediate value that represents the current animated value of the property.
      </p>

   <p>
        Only animatable CSS properties can be transitioned. See the table at the end of this document for a list
        of properties that are animatable.
      </p>

   <p>
        The transition for a property is defined using a number of new properties. For example:
      </p>

   <div class="example">

    <p style="display:none">
          Example(s):
        </p>


    <pre>div {
  transition-property: opacity;
  transition-duration: 2s;
}
</pre>
    The above example defines a transition on the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-color-3/#opacity">opacity</a> property that, when a new value is assigned to it, will cause a smooth change between the old value and the new value over a period of two seconds.
   </div>

   <p>
        Each of the transition properties accepts a comma-separated list, allowing multiple transitions to be defined, each acting on a different property. In this case, the individual transitions take their parameters from the same index in all the lists. For example:
      </p>

   <div class="example">

    <p style="display:none">
          Example(s):
        </p>


    <pre>div {
  transition-property: opacity, left;
  transition-duration: 2s, 4s;
}

</pre>
    This will cause the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-color-3/#opacity">opacity</a> property to transition over a period of two seconds and the left property to transition over a period of four seconds.
   </div>


   <p id="list-matching">
        In the case where the lists of values in transition properties
        do not have the same length, the length of the
        <a class="property" data-link-type="propdesc" href="#propdef-transition-property">transition-property</a> list determines the number of items in
        each list examined when starting transitions.  The lists are
        matched up from the first value: excess values at the end are
        not used.  If one of the other properties doesn’t have enough
        comma-separated values to match the number of values of
        <a class="property" data-link-type="propdesc" href="#propdef-transition-property">transition-property</a>, the UA must calculate its used value by
        repeating the list of values until there are enough.  This
        truncation or repetition does not affect the computed value.
        <span class="note" role="note">
          Note: This is analogous to the behavior of the <a class="property" data-link-type="propdesc">background-*</a>
          properties, with <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-backgrounds-3/#background-image">background-image</a> analogous to
          <a class="property" data-link-type="propdesc" href="#propdef-transition-property">transition-property</a>.
        </span>
      </p>


   <div class="example">

    <p style="display:none">
          Example(s):
        </p>


    <pre>div {
  transition-property: opacity, left, top, width;
  transition-duration: 2s, 1s;
}
</pre>
    The above example defines a transition on the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-color-3/#opacity">opacity</a> property of 2 seconds duration, a      transition on the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css2/visuren.html#propdef-left">left</a> property of 1
      second duration, a transition on the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css2/visuren.html#propdef-top">top</a> property of 2 seconds duration and a
      transition on the <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css2/visudet.html#propdef-width">width</a> property of 1
      second duration.


   </div>


   <p>
        While authors can use transitions to create dynamically changing content,
        dynamically changing content can lead to seizures in some users.
        For information on how to avoid content that can lead to seizures, see
        <a href="http://www.w3.org/TR/WCAG20/#seizure">Guideline 2.3:
        Seizures:
        Do not design content in a way that is known to cause seizures</a>
        (<a data-link-type="biblio" href="#biblio-wcag20">[WCAG20]</a>).
      </p>


   <h3 class="heading settled" data-level="2.1" id="transition-property-property"><span class="secno">2.1. </span><span class="content"><span id="the-transition-property-property-">
        The <a class="property" data-link-type="propdesc" href="#propdef-transition-property">transition-property</a> Property
      </span></span><a class="self-link" href="#transition-property-property"></a></h3>

   <p>
        The <a class="property" data-link-type="propdesc" href="#propdef-transition-property">transition-property</a> property specifies the name of the CSS property to which the transition is applied.
      </p>

   <table class="definition propdef">
    <tbody>
     <tr>
      <th>Name:
      <td><dfn class="css" data-dfn-type="property" data-export="" id="propdef-transition-property">transition-property<a class="self-link" href="#propdef-transition-property"></a></dfn>
     <tr>
      <th>Value:
      <td class="prod"><a class="css" data-link-type="maybe" href="#valdef-transition-property-none">none</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="production css" data-link-type="type" href="#single-transition-property">&lt;single-transition-property></a><a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#mult-comma">#</a>
     <tr>
      <th>Initial:
      <td><a class="css" data-link-type="maybe" href="#valdef-transition-property-all">all</a>
     <tr>
      <th>Applies to:
      <td>all elements, :before and :after pseudo elements
     <tr>
      <th>Inherited:
      <td>no
     <tr>
      <th>Percentages:
      <td>N/A
     <tr>
      <th>Media:
      <td>visual
     <tr>
      <th>Computed value:
      <td>Same as specified value.
     <tr>
      <th>Animatable:
      <td>no
     <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr></table>


   <div class="prod">
        <dfn class="css" data-dfn-type="type" data-export="" id="single-transition-property">&lt;single-transition-property><a class="self-link" href="#single-transition-property"></a></dfn> = <a class="css" data-link-type="maybe" href="#valdef-transition-property-all">all</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#identifier-value">&lt;custom-ident></a>;
      </div>


   <p>
        A value of
        <dfn class="css" data-dfn-for="transition-property" data-dfn-type="value" data-export="" id="valdef-transition-property-none">none<a class="self-link" href="#valdef-transition-property-none"></a></dfn>
        means that no property will transition.
        Otherwise, a list of properties to be transitioned, or the
        keyword <dfn class="css" data-dfn-for="transition-property" data-dfn-type="value" data-export="" id="valdef-transition-property-all">all<a class="self-link" href="#valdef-transition-property-all"></a></dfn>
        which indicates that all properties are to be
        transitioned, is given.
      </p>


   <p>
        If one of the identifiers listed is not a recognized property
        name or is not an animatable property, the implementation must
        still start transitions on the animatable properties in the
        list using the duration, delay, and timing function at their
        respective indices in the lists for <a class="property" data-link-type="propdesc" href="#propdef-transition-duration">transition-duration</a>,
        <a class="property" data-link-type="propdesc" href="#propdef-transition-delay">transition-delay</a>, and <a class="property" data-link-type="propdesc" href="#propdef-transition-timing-function">transition-timing-function</a>.  In other
        words, unrecognized or non-animatable properties must be kept in
        the list to preserve the matching of indices.
      </p>


   <p>
        The <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#identifier-value">&lt;custom-ident></a> production in <a class="production css" data-link-type="type" href="#single-transition-property">&lt;single-transition-property></a>
        also excludes the keyword <a class="css" data-link-type="maybe" href="#valdef-transition-property-none">none</a>,
        in addition to the keywords always excluded from <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#identifier-value">&lt;custom-ident></a>.
        This means that
        <a class="css" data-link-type="maybe" href="#valdef-transition-property-none">none</a>, <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-inherit">inherit</a>, and <a class="css" data-link-type="maybe" href="http://dev.w3.org/csswg/css-cascade-3/#valdef-all-initial">initial</a> are not
        permitted as items within a list of more that one identifier;
        any list that uses them is syntactically invalid.
      </p>


   <p>
        For the keyword <a class="css" data-link-type="maybe" href="#valdef-transition-property-all">all</a>,
        or if one of the identifiers listed is a
        shorthand property, implementations must start transitions for
        any of its longhand sub-properties that are animatable (or, for
        <a class="css" data-link-type="maybe" href="#valdef-transition-property-all">all</a>, all animatable properties), using the duration, delay,
        and timing function at the index corresponding to the shorthand.
      </p>

   <p>
        If a property is specified multiple times in the value of
        <a class="property" data-link-type="propdesc" href="#propdef-transition-property">transition-property</a> (either on its own, via a shorthand that
        contains it, or via the <a class="css" data-link-type="maybe" href="#valdef-transition-property-all">all</a> value), then the transition that
        starts uses the duration, delay, and timing function at the
        index corresponding to the <em>last</em> item in the value of
        <a class="property" data-link-type="propdesc" href="#propdef-transition-property">transition-property</a> that calls for animating that property.
      </p>

   <p class="note" role="note">
        Note:  The <a class="css" data-link-type="maybe" href="#valdef-transition-property-all">all</a> value and <a class="property" data-link-type="propdesc" href="http://dev.w3.org/csswg/css-cascade-3/#propdef-all">all</a> shorthand
        property work in similar ways, so the
        <a class="css" data-link-type="maybe" href="#valdef-transition-property-all">all</a> value is just like a shorthand that
        covers all properties.
      </p>


   <h3 class="heading settled" data-level="2.2" id="transition-duration-property"><span class="secno">2.2. </span><span class="content"><span id="the-transition-duration-property-">
        The <a class="property" data-link-type="propdesc" href="#propdef-transition-duration">transition-duration</a> Property
      </span></span><a class="self-link" href="#transition-duration-property"></a></h3>

   <p>
        The <a class="property" data-link-type="propdesc" href="#propdef-transition-duration">transition-duration</a> property defines the length of time that a transition takes.
      </p>

   <table class="definition propdef">
    <tbody>
     <tr>
      <th>Name:
      <td><dfn class="css" data-dfn-type="property" data-export="" id="propdef-transition-duration">transition-duration<a class="self-link" href="#propdef-transition-duration"></a></dfn>
     <tr>
      <th>Value:
      <td class="prod"><a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#time-value">&lt;time></a><a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#mult-comma">#</a>
     <tr>
      <th>Initial:
      <td><span class="css">0s</span>
     <tr>
      <th>Applies to:
      <td>all elements, :before and :after pseudo elements
     <tr>
      <th>Inherited:
      <td>no
     <tr>
      <th>Percentages:
      <td>N/A
     <tr>
      <th>Media:
      <td>interactive
     <tr>
      <th>Computed value:
      <td>Same as specified value.
     <tr>
      <th>Animatable:
      <td>no
     <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr></table>

   <p>
        This property specifies how long the transition from the old value to the new value should take. By default the value is <span class="css">0s</span>, meaning that the transition is immediate (i.e. there will be no animation). A negative value for <a class="property" data-link-type="propdesc" href="#propdef-transition-duration">transition-duration</a> renders the declaration invalid.
      </p>


   <h3 class="heading settled" data-level="2.3" id="transition-timing-function-property"><span class="secno">2.3. </span><span class="content"><span id="transition-timing-function_tag">
        The <a class="property" data-link-type="propdesc" href="#propdef-transition-timing-function">transition-timing-function</a> Property
      </span></span><a class="self-link" href="#transition-timing-function-property"></a></h3>

   <p>
        The <a class="property" data-link-type="propdesc" href="#propdef-transition-timing-function">transition-timing-function</a> property
        describes how the intermediate values used during a transition will be
        calculated. It allows for a transition to change speed over its
        duration. These effects are commonly called <em>easing</em> functions.
        In either case, a mathematical function that provides a smooth curve is
        used.
      </p>

   <p>
        Timing functions are either defined as a stepping function or
        a <a href="http://en.wikipedia.org/wiki/B%C3%A9zier_curve#Cubic_B.C3.A9zier_curves">cubic
        Bézier curve</a>.
        The timing function takes as its input
        the current elapsed percentage of the transition duration
        and outputs the percentage of the way the transition is
        from its start value to its end value.
        How this output is used is defined by
        the <a href="#animatable-types">interpolation rules</a>
        for the value type.
      </p>

   <p>
        A <a href="http://en.wikipedia.org/wiki/Step_function">stepping</a>
        function is defined by a number that divides the domain of operation
        into equally sized intervals. Each subsequent interval is a equal step
        closer to the goal state. The function also specifies whether the
        change in output percentage happens at the start or end of the
        interval (in other words, if 0% on the input percentage is the point
        of initial change).
      </p>

   <div class="figure">
        <img alt="The step timing function splits
          the function domain into a number of disjoint straight line
          segments. steps(1, start) is a function whose
          output value is 1 for all input values. steps(1, end) is a function whose
          output value is 0 for all input values less than 1, and output
          is 1 for the input value of 1. steps(3, start) is a function that
          divides the input domain into three segments, each 1/3 in length,
          and 1/3 above the previous segment, with the first segment starting
          at 1/3. steps(3, end) is a function that
          divides the input domain into three segments, each 1/3 in length,
          and 1/3 above the previous segment, with the first segment starting
          at 0." src="step.png">
      </div>

   <p class="caption">
        Step timing functions
      </p>

   <p>
        A <a href="http://en.wikipedia.org/wiki/B%C3%A9zier_curve#Cubic_B.C3.A9zier_curves">cubic
        Bézier curve</a> is defined by four control points, P<sub>0</sub>
        through P<sub>3</sub> (see Figure 1). P<sub>0</sub> and P<sub>3</sub>
        are always set to (0,0) and (1,1). The <a class="property" data-link-type="propdesc" href="#propdef-transition-timing-function">transition-timing-function</a> property is used
        to specify the values for points P<sub>1</sub> and P<sub>2</sub>. These
        can be set to preset values using the keywords listed below, or can be
        set to specific values using the <a class="css" data-link-type="maybe" href="#funcdef-cubic-bezier">cubic-bezier()</a> function.
        In the <a class="css" data-link-type="maybe" href="#funcdef-cubic-bezier">cubic-bezier()</a> function, P<sub>1</sub> and
        P<sub>2</sub> are each specified by both an X and Y value.
      </p>

   <div class="figure">
        <img alt="The Bézier timing function is a
          smooth curve from point P0 = (0,0) to point P3 = (1,1). The
          length and orientation of the line segment P0-P1 determines
          the tangent and the curvature of the curve at P0 and the
          line segment P2-P3 does the same at P3." src="TimingFunction.png">
      </div>

   <p class="caption">
        Bézier Timing Function Control Points
      </p>

   <table class="definition propdef">
    <tbody>
     <tr>
      <th>Name:
      <td><dfn class="css" data-dfn-type="property" data-export="" id="propdef-transition-timing-function">transition-timing-function<a class="self-link" href="#propdef-transition-timing-function"></a></dfn>
     <tr>
      <th>Value:
      <td class="prod"><a class="production css" data-link-type="type" href="#single-transition-timing-function">&lt;single-transition-timing-function></a><a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#mult-comma">#</a>
     <tr>
      <th>Initial:
      <td><a class="css" data-link-type="maybe" href="#valdef-transition-timing-function-ease">ease</a>
     <tr>
      <th>Applies to:
      <td>all elements, :before and :after pseudo elements
     <tr>
      <th>Inherited:
      <td>no
     <tr>
      <th>Percentages:
      <td>N/A
     <tr>
      <th>Media:
      <td>interactive
     <tr>
      <th>Computed value:
      <td>Same as specified value.
     <tr>
      <th>Animatable:
      <td>no
     <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr></table>

   <div class="prod">
        <dfn class="css" data-dfn-type="type" data-export="" id="single-transition-timing-function">&lt;single-transition-timing-function><a class="self-link" href="#single-transition-timing-function"></a></dfn> = <a class="css" data-link-type="maybe" href="#valdef-transition-timing-function-ease">ease</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="css" data-link-type="maybe" href="#valdef-transition-timing-function-linear">linear</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="css" data-link-type="maybe" href="#valdef-transition-timing-function-ease-in">ease-in</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="css" data-link-type="maybe" href="#valdef-transition-timing-function-ease-out">ease-out</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="css" data-link-type="maybe" href="#valdef-transition-timing-function-ease-in-out">ease-in-out</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="css" data-link-type="maybe" href="#valdef-transition-timing-function-step-start">step-start</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="css" data-link-type="maybe" href="#valdef-transition-timing-function-step-end">step-end</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="css" data-link-type="function" href="#funcdef-steps">steps</a>(<a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#integer-value">&lt;integer></a>[<a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-comma">,</a> [ <a class="css" data-link-type="maybe" href="#valdef-steps-start">start</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="css" data-link-type="maybe" href="#valdef-steps-end">end</a> ] ]<a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#mult-opt">?</a>) <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="css" data-link-type="function" href="#funcdef-cubic-bezier">cubic-bezier</a>(<a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#number-value">&lt;number></a><a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-comma">,</a> <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#number-value">&lt;number></a><a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-comma">,</a> <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#number-value">&lt;number></a><a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-comma">,</a> <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#number-value">&lt;number></a>)
      </div>

   <p>
        The timing functions have the following definitions.
      </p>

   <dl>

    <dt><dfn class="css" data-dfn-for="transition-timing-function" data-dfn-type="value" data-export="" id="valdef-transition-timing-function-ease">ease<a class="self-link" href="#valdef-transition-timing-function-ease"></a></dfn>


    <dd>
          The ease function is equivalent to <a class="css" data-link-type="function" href="#funcdef-cubic-bezier">cubic-bezier(0.25, 0.1, 0.25, 1)</a>.


    <dt><dfn class="css" data-dfn-for="transition-timing-function" data-dfn-type="value" data-export="" id="valdef-transition-timing-function-linear">linear<a class="self-link" href="#valdef-transition-timing-function-linear"></a></dfn>


    <dd>
          The linear function is equivalent to <a class="css" data-link-type="function" href="#funcdef-cubic-bezier">cubic-bezier(0, 0, 1, 1)</a>.


    <dt><dfn class="css" data-dfn-for="transition-timing-function" data-dfn-type="value" data-export="" id="valdef-transition-timing-function-ease-in">ease-in<a class="self-link" href="#valdef-transition-timing-function-ease-in"></a></dfn>


    <dd>
          The ease-in function is equivalent to <a class="css" data-link-type="function" href="#funcdef-cubic-bezier">cubic-bezier(0.42, 0, 1, 1)</a>.


    <dt><dfn class="css" data-dfn-for="transition-timing-function" data-dfn-type="value" data-export="" id="valdef-transition-timing-function-ease-out">ease-out<a class="self-link" href="#valdef-transition-timing-function-ease-out"></a></dfn>


    <dd>
          The ease-out function is equivalent to <a class="css" data-link-type="function" href="#funcdef-cubic-bezier">cubic-bezier(0, 0, 0.58, 1)</a>.


    <dt><dfn class="css" data-dfn-for="transition-timing-function" data-dfn-type="value" data-export="" id="valdef-transition-timing-function-ease-in-out">ease-in-out<a class="self-link" href="#valdef-transition-timing-function-ease-in-out"></a></dfn>


    <dd>
          The ease-in-out function is equivalent to <a class="css" data-link-type="function" href="#funcdef-cubic-bezier">cubic-bezier(0.42, 0, 0.58, 1)</a>.


    <dt><dfn class="css" data-dfn-for="transition-timing-function" data-dfn-type="value" data-export="" id="valdef-transition-timing-function-step-start">step-start<a class="self-link" href="#valdef-transition-timing-function-step-start"></a></dfn>


    <dd>
          The step-start function i
108AF
s equivalent to <a class="css" data-link-type="function" href="#funcdef-steps">steps(1, start)</a>.


    <dt><dfn class="css" data-dfn-for="transition-timing-function" data-dfn-type="value" data-export="" id="valdef-transition-timing-function-step-end">step-end<a class="self-link" href="#valdef-transition-timing-function-step-end"></a></dfn>


    <dd>
          The step-end function is equivalent to <a class="css" data-link-type="function" href="#funcdef-steps">steps(1, end)</a>.


    <dt><dfn class="css" data-dfn-for="transition-timing-function" data-dfn-type="function" data-export="" data-lt="steps()" id="funcdef-steps">steps(<a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#integer-value">&lt;integer></a>[, [ start | end ] ]?)<a class="self-link" href="#funcdef-steps"></a></dfn>


    <dd>
          Specifies a stepping function, described above, taking two
          parameters. The first parameter specifies the number of intervals
          in the function. It must be a positive integer (greater than 0).
          The second parameter, which is optional, is
          either the value <dfn class="css" data-dfn-for="steps()" data-dfn-type="value" data-export="" id="valdef-steps-start">start<a class="self-link" href="#valdef-steps-start"></a></dfn> or <dfn class="css" data-dfn-for="steps()" data-dfn-type="value" data-export="" id="valdef-steps-end">end<a class="self-link" href="#valdef-steps-end"></a></dfn>, and specifies the point
          at which the change of values occur within the interval.
          If the second parameter is omitted, it is given the value <a class="css" data-link-type="maybe" href="#valdef-steps-end">end</a>.


    <dt><dfn class="css" data-dfn-for="transition-timing-function" data-dfn-type="function" data-export="" data-lt="cubic-bezier()" id="funcdef-cubic-bezier">cubic-bezier(<a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#number-value">&lt;number></a>, <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#number-value">&lt;number></a>, <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#number-value">&lt;number></a>, <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#number-value">&lt;number></a>)<a class="self-link" href="#funcdef-cubic-bezier"></a></dfn>


    <dd>
          Specifies a <a href="http://en.wikipedia.org/wiki/B%C3%A9zier_curve">cubic-bezier
          curve</a>. The four values specify points P<sub>1</sub> and
          P<sub>2</sub> of the curve as (x1, y1, x2, y2). Both x values must be
          in the range [0, 1] or the definition is invalid. The y values can
          exceed this range.


   </dl>

   <h3 class="heading settled" data-level="2.4" id="transition-delay-property"><span class="secno">2.4. </span><span class="content"><span id="the-transition-delay-property-">
        The <a class="property" data-link-type="propdesc" href="#propdef-transition-delay">transition-delay</a> Property
      </span></span><a class="self-link" href="#transition-delay-property"></a></h3>

   <p>
        The <a class="property" data-link-type="propdesc" href="#propdef-transition-delay">transition-delay</a> property defines when the transition will start. It allows a transition to begin execution some some period of time from when it is applied. A <a class="property" data-link-type="propdesc" href="#propdef-transition-delay">transition-delay</a> value of <span class="css">0s</span> means the transition will execute as soon as the property is changed. Otherwise, the value specifies an offset from the moment the property is changed, and the transition will delay execution by that offset.
      </p>

   <p>
        If the value for <a class="property" data-link-type="propdesc" href="#propdef-transition-delay">transition-delay</a> is a negative time offset then the transition will execute the moment the property is changed, but will appear to have begun execution at the specified offset. That is, the transition will appear to begin part-way through its play cycle. In the case where a transition has implied starting values and a negative <a class="property" data-link-type="propdesc" href="#propdef-transition-delay">transition-delay</a>, the starting values are taken from the moment the property is changed.
      </p>

   <table class="definition propdef">
    <tbody>
     <tr>
      <th>Name:
      <td><dfn class="css" data-dfn-type="property" data-export="" id="propdef-transition-delay">transition-delay<a class="self-link" href="#propdef-transition-delay"></a></dfn>
     <tr>
      <th>Value:
      <td class="prod"><a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#time-value">&lt;time></a><a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#mult-comma">#</a>
     <tr>
      <th>Initial:
      <td><span class="css">0s</span>
     <tr>
      <th>Applies to:
      <td>all elements, :before and :after pseudo elements
     <tr>
      <th>Inherited:
      <td>no
     <tr>
      <th>Percentages:
      <td>N/A
     <tr>
      <th>Media:
      <td>interactive
     <tr>
      <th>Computed value:
      <td>Same as specified value.
     <tr>
      <th>Animatable:
      <td>no
     <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr></table>
   <h3 class="heading settled" data-level="2.5" id="transition-shorthand-property"><span class="secno">2.5. </span><span class="content"><span id="the-transition-shorthand-property-">
        The <a class="property" data-link-type="propdesc" href="#propdef-transition">transition</a> Shorthand Property
      </span></span><a class="self-link" href="#transition-shorthand-property"></a></h3>

   <p>
        The <a class="property" data-link-type="propdesc" href="#propdef-transition">transition</a> shorthand property combines the four properties described above into a single property.
      </p>

   <table class="definition propdef">
    <tbody>
     <tr>
      <th>Name:
      <td><dfn class="css" data-dfn-type="property" data-export="" id="propdef-transition">transition<a class="self-link" href="#propdef-transition"></a></dfn>
     <tr>
      <th>Value:
      <td class="prod"><a class="production css" data-link-type="type" href="#single-transition">&lt;single-transition></a><a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#mult-comma">#</a>
     <tr>
      <th>Initial:
      <td>see individual properties
     <tr>
      <th>Applies to:
      <td>all elements, :before and :after pseudo elements
     <tr>
      <th>Inherited:
      <td>no
     <tr>
      <th>Percentages:
      <td>N/A
     <tr>
      <th>Media:
      <td>interactive
     <tr>
      <th>Computed value:
      <td>see individual properties
     <tr>
      <th>Animatable:
      <td>no
     <tr>
      <th>Canonical order:
      <td><abbr title="follows order of property value definition">per grammar</abbr></table>


   <div class="prod">
        <dfn class="css" data-dfn-type="type" data-export="" id="single-transition">&lt;single-transition><a class="self-link" href="#single-transition"></a></dfn> = [ <a class="css" data-link-type="maybe" href="#valdef-transition-property-none">none</a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-one">|</a> <a class="production css" data-link-type="type" href="#single-transition-property">&lt;single-transition-property></a> ] <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-any">||</a> <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#time-value">&lt;time></a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-any">||</a> <a class="production css" data-link-type="type" href="#single-transition-timing-function">&lt;single-transition-timing-function></a> <a data-link-type="grammar" href="http://dev.w3.org/csswg/css-values-3/#comb-any">||</a> <a class="production css" data-link-type="type" href="http://dev.w3.org/csswg/css-values-3/#time-value">&lt;time></a>
      </div>


   <p>
        Note that order is important within the items in this property:
        the first value that can be parsed as a time is assigned to the
        transition-duration,
        and the second value that can be parsed as a time is assigned to
        transition-delay.
      </p>


   <p>
        If there is more than one <a class="production css" data-link-type="type" href="#single-transition">&lt;single-transition></a> in the shorthand,
        and any of the transitions has
        <a class="css" data-link-type="maybe" href="#valdef-transition-property-none">none</a> as the <a class="production css" data-link-type="type" href="#single-transition-property">&lt;single-transition-property></a>,
        then the declaration is invalid.
      </p>


   <h2 class="heading settled" data-level="3" id="starting"><span class="secno">3. </span><span class="content">
        Starting of transitions
      </span><a class="self-link" href="#starting"></a></h2>


   <p>
        Implementations must maintain a set of
        <dfn data-dfn-type="dfn" data-lt="running transition" data-noexport="" id="running-transition">running transitions<a class="self-link" href="#running-transition"></a></dfn>,
        each of which applies to a specific element and non-shorthand
        property.  Each of these transitions also has a
        <dfn data-dfn-type="dfn" data-noexport="" id="start-time">start time<a class="self-link" href="#start-time"></a></dfn>, <dfn data-dfn-type="dfn" data-noexport="" id="end-time">end time<a class="self-link" href="#end-time"></a></dfn>, <dfn data-dfn-type="dfn" data-noexport="" id="start-value">start value<a class="self-link" href="#start-value"></a></dfn>,
        <dfn data-dfn-type="dfn" data-noexport="" id="end-value">end value<a class="self-link" href="#end-value"></a></dfn>, <dfn data-dfn-type="dfn" data-noexport="" id="reversing_adjusted-start-value">reversing-adjusted start value<a class="self-link" href="#reversing_adjusted-start-value"></a></dfn>,
        and <dfn data-dfn-type="dfn" data-noexport="" id="reversing-shortening-factor">reversing shortening factor<a class="self-link" href="#reversing-shortening-factor"></a></dfn>.
        Transitions are added to this set as described in this section,
        and are removed from this set
        when they <a data-link-type="dfn" href="#dfn-complete">complete</a>
        or when implementations are required to <dfn data-dfn-type="dfn" data-noexport="" id="cancel">cancel<a class="self-link" href="#cancel"></a></dfn> them.
        <span class="note" role="note">
          For the rationale behind the <a data-link-type="dfn" href="#reversing_adjusted-start-value">reversing-adjusted start value</a>
          and <a data-link-type="dfn" href="#reversing-shortening-factor">reversing shortening factor</a>, see <a href="#reversing">§3.1
        Faster reversing of interrupted transitions
      </a>.
        </span>
      </p>


   <p>
        Implementations must also maintain a set of
        <dfn data-dfn-type="dfn" data-lt="completed transition" data-noexport="" id="completed-transition">completed transitions<a class="self-link" href="#completed-transition"></a></dfn>,
        each of which
        (like <a data-link-type="dfn" href="#running-transition">running transitions</a>)
        applies to a specific element and non-shorthand property.
        <span class="note" role="note">
          This specification maintains the invariant that
          there is never both a <a data-link-type="dfn" href="#running-transition">running transition</a> and
          a <a data-link-type="dfn" href="#completed-transition">completed transition</a> for the same property and element.
        </span>
      </p>


   <p>
        If an element is no longer in the document,
        implementations must remove transitions on it
        from the <a data-link-type="dfn" href="#running-transition">running transitions</a> and the
        <a data-link-type="dfn" href="#completed-transition">completed transitions</a>.
      </p>


   <div class="note" role="note">


    <p>
          This set of completed transitions
          needs to be maintained
          in order to prevent
          transitions from repeating themselves in certain cases,
          i.e., to maintain the invariant
          that this specification tries to maintain
          that unrelated style changes do not trigger transitions.
        </p>


    <p class="example">
          An example where maintaining the set of completed transitions
          is necessary would be a transition on
          an inherited property,
          where the parent specifies a transition of that property for
          a longer duration (say, <a class="css" data-link-type="propdesc" href="#propdef-transition">transition: 4s text-indent</a>)
          and a child element that inherits the parent’s value specifies
          a transition of the same property for a shorter duration
          (say, <a class="css" data-link-type="propdesc" href="#propdef-transition">transition: 1s text-indent</a>).
          Without the maintenance of this set of completed transitions,
          implementations could start additional transitions on the child
          after the initial 1 second transition on the child completes.
        </p>


   </div>


   <p>
        Various things can cause the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-cascade-3/#computed-value">computed values</a> of properties
        on an element to change.
        These include
        insertion and removal of elements from the document tree
        (which both changes whether those elements have <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-cascade-3/#computed-value">computed values</a> and
        can change the styles of other elements through selector matching),
        changes to the document tree that cause
        changes to which selectors match elements,
        changes to style sheets or style attributes,
        and other things.
        This specification does not define when <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-cascade-3/#computed-value">computed values</a> are updated,
        beyond saying that implementations must not
        use, present, or display something resulting from the CSS
        cascading, value computation, and inheritance process <a data-link-type="biblio" href="#biblio-css3cascade">[CSS3CASCADE]</a>
        without updating the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-cascade-3/#computed-value">computed value</a>
        (which means merely that implementations cannot avoid
        meeting requirements of this specification
        by claiming not to have updated the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-cascade-3/#computed-value">computed value</a>
        as part of handling a style change).
        However,
        when an implementation updates the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-cascade-3/#computed-value">computed value</a> of a
        property on an element
        to reflect one of these changes,
        or computes the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-cascade-3/#computed-value">computed value</a> of a property on an element
        newly added to the document,
        it must update the <a data-link-type="dfn" href="http://dev.w3.org/csswg/css-cascade-3/#computed-value">computed value</a>
        for all properties and elements to reflect all
        of these changes at the same time
        (or at least it must be undetectable that it was done at a
        different time).
        This processing of a set of simultaneous style changes is called a
        <dfn data-dfn-type="dfn" data-noexport="" id="style-change-event">style change event<a class="self-link" href="#style-change-event"></a></dfn>.
        (Implementations typically have a <a data-link-type="dfn" href="#style-change-event">style change event</a> to
        correspond with their desired screen refresh rate,
        and when up-to-date computed style or layout information is needed
        for a script API that depends on it.)
      </p>


   <p>
        Since this specification does not define
        when a <a data-link-type="dfn" href="#style-change-event">style change event</a> occurs,