csswg-drafts/css-transitions/Overview.src.html at cb7761df9700c53f1d51397ece47acbf3dc5429a · w3c/csswg-drafts · GitHub

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  'http://www.w3.org/TR/html4/strict.dtd'>

<html lang="en">
<head>
  <title>CSS Transitions</title>
  <script src='http://test.csswg.org/harness/annotate.js#css-transitions-1_dev' type='text/javascript' defer></script>
  <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
  <link rel="stylesheet" type="text/css" href="../default.css">
  <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>
  <link rel="stylesheet" type="text/css" href="http://www.w3.org/StyleSheets/TR/W3C-[STATUS].css">
</head>

<body>

<div class="head">
<!--logo-->

<h1>CSS Transitions</h1>

<h2 class="no-num no-toc">[LONGSTATUS] [DATE]</h2>
<dl>
  <dt>This version:
    <dd>
    <a href="[VERSION]">[VERSION]</a>
  <dt>Latest version:
    <dd><a href="http://www.w3.org/TR/css3-transitions/">
      [LATEST]</a>
  <dt>Editor's draft:
    <dd><a href="http://dev.w3.org/csswg/[SHORTNAME]/">http://dev.w3.org/csswg/[SHORTNAME]/</a>
    (<a href="https://dvcs.w3.org/hg/csswg/log/tip/css-transitions/Overview.src.html">change log</a>,
    <a href="https://dvcs.w3.org/hg/csswg/log/tip/css3-transitions/Overview.src.html">older change log</a>)
  <dt>Previous version:
    <dd><a href="http://www.w3.org/TR/2013/WD-css3-transitions-20130212/">http://www.w3.org/TR/2013/WD-css3-transitions-20130212/</a>
  <dt id="editors-list">Editors:
    <dd><a href="mailto:dino@apple.com">Dean Jackson</a> (<a
      href="http://www.apple.com/">Apple Inc</a>)
    <dd><a href="mailto:hyatt@apple.com">David Hyatt</a> (<a
      href="http://www.apple.com/">Apple Inc</a>)
    <dd><a href="mailto:cmarrin@apple.com">Chris Marrin</a> (<a
      href="http://www.apple.com/">Apple Inc</a>)
    <dd class=vcard><a class=fn href="http://dbaron.org/">L. David Baron</a> (<a
      class=org href="http://www.mozilla.org/">Mozilla</a>)

  <dt>Issues list:
    <dd><a href="https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&amp;product=CSS&amp;component=Transitions&amp;resolution=---&amp;cmdtype=doit">in Bugzilla</a>

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

  <dt>Test suite:
    <dd>  <a href="http://test.csswg.org/suites/css-transitions-1/nightly-unstable/">http://test.csswg.org/suites/css-transitions-1/nightly-unstable/</a>
</dl>

<!--copyright-->

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

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

<p>CSS Transitions allows property changes in CSS values to occur smoothly
  over a specified duration.

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

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


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

      <p><em>This section is not normative.</em>
      <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 id="transitions"><a id="transitions-">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 'left' and
        'background-color' 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 src="transition1.png" alt="">
      </div>
      <p class="caption">
        Transitions of 'left' and 'background-color'
      </p>
      <p>
        Transitions are a presentational effect. The computed value of a property transitions over time from the old value to the new value. Therefore if a script queries the computed style of a property 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 'opacity' 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 'opacity' 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
        'transition-property' 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
        'transition-property', 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">
          Note: This is analogous to the behavior of the 'background-*'
          properties, with 'background-image' analogous to
          'transition-property'.
        </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 'opacity' property of 2 seconds duration, a
      transition on the 'left' property of 1
      second duration, a transition on the 'top' property of 2 seconds duration and a
      transition on the 'width' 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>
        ([[WCAG20]]).
      </p>

      <!-- ======================================================================================================= -->
      <h3 id="transition-property-property"><a id="the-transition-property-property-">
        The 'transition-property' Property
      </a></h3>
      <p>
        The 'transition-property' property specifies the name of the CSS property to which the transition is applied.
      </p>
      <table class="propdef">
        <tbody>
          <tr>
            <td>
              <em>Name:</em>
            </td>
            <td>
              <dfn id="transition-property">transition-property</dfn>
            </td>
          </tr>
          <tr>
            <td>
              <em>Value:</em>
            </td>
            <td>
              none | <span>&lt;single-transition-property&gt;</span> [ ',' <span>&lt;single-transition-property&gt;</span> ]*
            </td>
          </tr>
          <tr>
            <td>
              <em>Initial:</em>
            </td>
            <td>
              all
            </td>
          </tr>
          <tr>
            <td>
              <em>Applies to:</em>
            </td>
            <td>
              all elements, :before and :after pseudo elements
            </td>
          </tr>
          <tr>
            <td>
              <em>Inherited:</em>
            </td>
            <td>
              no
            </td>
          </tr>
          <tr>
            <td>
              <em>Animatable:</em>
            </td>
            <td>
              no
            </td>
          </tr>
          <tr>
            <td>
              <em>Percentages:</em>
            </td>
            <td>
              N/A
            </td>
          </tr>
          <tr>
            <td>
              <em>Media:</em>
            </td>
            <td>
            </td>
          </tr>
          <tr>
            <td>
              <em>Computed value:</em>
            </td>
            <td>
              Same as specified value.
            </td>
          </tr>
          <tr>
            <td>
              <em>Canonical order:</em>
            </td>
            <td>
              <abbr title="follows order of property value definition">per grammar</abbr>
            </td>
          </tr>
        </tbody>
      </table>

      <div class="prod">
        <dfn id="single-transition-property">&lt;single-transition-property&gt;</dfn> = all | &lt;IDENT&gt;
      </div>

      <p>
        A value of ''none'' means that no property will transition.
        Otherwise, a list of properties to be transitioned, or the
        keyword ''all'' 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 'transition-duration',
        'transition-delay', and 'transition-timing-function'.  In other
        words, unrecognized or non-animatable properties must be kept in
        the list to preserve the matching of indices.
      </p>

      <p>
        The keywords ''none'', ''inherit'', and ''initial'' are not
        permitted as items within a list of more that one identifier;
        any list that uses them is syntactically invalid.
        In other words, the &lt;IDENT&gt; production in
        <span>&lt;single-transition-property&gt;</span> matches any
        identifier other than these three keywords.
      </p>

      <p>
        For the keyword ''all'', 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
        ''all'', 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
        'transition-property' (either on its own, via a shorthand that
        contains it, or via the ''all'' 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
        'transition-property' that calls for animating that property.
      </p>
      <p class="note">
        Note:  The ''all'' value and 'all' shorthand
        property work in similar ways, so the
        ''all'' value is just like a shorthand that
        covers all properties.
      </p>

      <!-- ======================================================================================================= -->
      <h3 id="transition-duration-property"><a id="the-transition-duration-property-">
        The 'transition-duration' Property
      </a></h3>
      <p>
        The 'transition-duration' property defines the length of time that a transition takes.
      </p>
      <table class="propdef">
        <tbody>
          <tr>
            <td>
              <em>Name:</em>
            </td>
            <td>
              <dfn id="transition-duration">transition-duration</dfn>
            </td>
          </tr>
          <tr>
            <td>
              <em>Value:</em>
            </td>
            <td>
              <span>&lt;time&gt;</span> [, <span>&lt;time&gt;</span>]*
            </td>
          </tr>
          <tr>
            <td>
              <em>Initial:</em>
            </td>
            <td>
              0s
            </td>
          </tr>
          <tr>
            <td>
              <em>Applies to:</em>
            </td>
            <td>
              all elements, :before and :after pseudo elements
            </td>
          </tr>
          <tr>
            <td>
              <em>Inherited:</em>
            </td>
            <td>
              no
            </td>
          </tr>
          <tr>
            <td>
              <em>Animatable:</em>
            </td>
            <td>
              no
            </td>
          </tr>
            <td>
              <em>Percentages:</em>
            </td>
            <td>
              N/A
            </td>
          </tr>
          <tr>
            <td>
              <em>Media:</em>
            </td>
            <td>
              interactive
            </td>
          </tr>
          <tr>
            <td>
              <em>Computed value:</em>
            </td>
            <td>
              Same as specified value.
            </td>
          </tr>
          <tr>
            <td>
              <em>Canonical order:</em>
            </td>
            <td>
              <abbr title="follows order of property value definition">per grammar</abbr>
            </td>
          </tr>
        </tbody>
      </table>
      <p>
        This property specifies how long the transition from the old value to the new value should take. By default the value is ''0s'', meaning that the transition is immediate (i.e. there will be no animation). A negative value for 'transition-duration' renders the declaration invalid.
      </p>

      <!-- =======================================================================================================
        -->

      <h3 id="transition-timing-function-property"><a id="transition-timing-function_tag">
        The 'transition-timing-function' Property
      </a></h3>
      <p>
        The 'transition-timing-function' 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
        B&eacute;zier 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 src="step.png" 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.">
      </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
        B&eacute;zier 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 'transition-timing-function' 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 ''cubic-bezier'' function.
        In the ''cubic-bezier'' 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 src="TimingFunction.png" alt="The B&eacute;zier 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.">
      </div>
      <p class="caption">
        B&eacute;zier Timing Function Control Points
      </p>
      <table class="propdef">
        <tbody>
          <tr>
            <td>
              <em>Name:</em>
            </td>
            <td>
              <dfn id="transition-timing-function">transition-timing-function</dfn>
            </td>
          </tr>
          <tr>
            <td>
              <em>Value:</em>
            </td>
            <td>
              <span>&lt;single-transition-timing-function&gt;</span> [ ',' <span>&lt;single-transition-timing-function&gt;</span> ]*
            </td>
          </tr>
          <tr>
            <td>
              <em>Initial:</em>
            </td>
            <td>
              ease
            </td>
          </tr>
          <tr>
            <td>
              <em>Applies to:</em>
            </td>
            <td>
              all elements, :before and :after pseudo elements
            </td>
          </tr>
          <tr>
            <td>
              <em>Inherited:</em>
            </td>
            <td>
              no
            </td>
          </tr>
          <tr>
            <td>
              <em>Animatable:</em>
            </td>
            <td>
              no
            </td>
          </tr>
          <tr>
            <td>
              <em>Percentages:</em>
            </td>
            <td>
              N/A
            </td>
          </tr>
          <tr>
            <td>
              <em>Media:</em>
            </td>
            <td>
              interactive
            </td>
          </tr>
          <tr>
            <td>
              <em>Computed value:</em>
            </td>
            <td>
              Same as specified value.
            </td>
          </tr>
          <tr>
            <td>
              <em>Canonical order:</em>
            </td>
            <td>
              <abbr title="follows order of property value definition">per grammar</abbr>
            </td>
          </tr>
        </tbody>
      </table>
      <div class="prod">
        <dfn id="single-transition-timing-function">&lt;single-transition-timing-function&gt;</dfn> = ease | linear | ease-in | ease-out | ease-in-out | step-start | step-end | steps(&lt;integer&gt;[, [ start | end ] ]?) | cubic-bezier(&lt;number&gt;, &lt;number&gt;, &lt;number&gt;, &lt;number&gt;)
      </div>
      <p>
        The timing functions have the following definitions.
      </p>
      <dl>
        <dt>
          ease
        </dt>
        <dd>
          The ease function is equivalent to cubic-bezier(0.25, 0.1, 0.25, 1).
        </dd>
        <dt>
          linear
        </dt>
        <dd>
          The linear function is equivalent to cubic-bezier(0, 0, 1, 1).
        </dd>
        <dt>
          ease-in
        </dt>
        <dd>
          The ease-in function is equivalent to cubic-bezier(0.42, 0, 1, 1).
        </dd>
        <dt>
          ease-out
        </dt>
        <dd>
          The ease-out function is equivalent to cubic-bezier(0, 0, 0.58, 1).
        </dd>
        <dt>
          ease-in-out
        </dt>
        <dd>
          The ease-in-out function is equivalent to cubic-bezier(0.42, 0, 0.58, 1)
        </dd>
        <dt>
          step-start
        </dt>
        <dd>
          The step-start function is equivalent to steps(1, start).
        </dd>
        <dt>
          step-end
        </dt>
        <dd>
          The step-end function is equivalent to steps(1, end).
        </dd>
        <dt>
          steps(&lt;integer&gt;[, [ start | end ] ]?)
        </dt>
        <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 ''start'' or ''end'', 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 'end'.
        </dd>
        <dt>
          cubic-bezier(&lt;number&gt;, &lt;number&gt;, &lt;number&gt;, &lt;number&gt;)
        
8338
</dt>
        <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.
        </dd>
      </dl><!-- ======================================================================================================= -->
      <h3 id="transition-delay-property"><a id="the-transition-delay-property-">
        The 'transition-delay' Property
      </a></h3>
      <p>
        The 'transition-delay' 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 'transition-delay' value of ''0s'' 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 'transition-delay' 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 'transition-delay', the starting values are taken from the moment the property is changed.
      </p>
      <table class="propdef">
        <tbody>
          <tr>
            <td>
              <em>Name:</em>
            </td>
            <td>
              <dfn id="transition-delay">transition-delay</dfn>
            </td>
          </tr>
          <tr>
            <td>
              <em>Value:</em>
            </td>
            <td>
              <span>&lt;time&gt;</span> [, <span>&lt;time&gt;</span>]*
            </td>
          </tr>
          <tr>
            <td>
              <em>Initial:</em>
            </td>
            <td>
              0s
            </td>
          </tr>
          <tr>
            <td>
              <em>Applies to:</em>
            </td>
            <td>
              all elements, :before and :after pseudo elements
            </td>
          </tr>
          <tr>
            <td>
              <em>Inherited:</em>
            </td>
            <td>
              no
            </td>
          </tr>
          <tr>
            <td>
              <em>Animatable:</em>
            </td>
            <td>
              no
            </td>
          </tr>
          <tr>
            <td>
              <em>Percentages:</em>
            </td>
            <td>
              N/A
            </td>
          </tr>
          <tr>
            <td>
              <em>Media:</em>
            </td>
            <td>
              interactive
            </td>
          </tr>
          <tr>
            <td>
              <em>Computed value:</em>
            </td>
            <td>
              Same as specified value.
            </td>
          </tr>
          <tr>
            <td>
              <em>Canonical order:</em>
            </td>
            <td>
              <abbr title="follows order of property value definition">per grammar</abbr>
            </td>
          </tr>
        </tbody>
      </table><!-- ======================================================================================================= -->
      <h3 id="transition-shorthand-property"><a id="the-transition-shorthand-property-">
        The 'transition' Shorthand Property
      </a></h3>
      <p>
        The 'transition' shorthand property combines the four properties described above into a single property.
      </p>
      <table class="propdef">
        <tbody>
          <tr>
            <td>
              <em>Name:</em>
            </td>
            <td>
              <dfn id="transition">transition</dfn>
            </td>
          </tr>
          <tr>
            <td>
              <em>Value:</em>
            </td>
            <td>
              <span>&lt;single-transition&gt;</span> [ ',' <span>&lt;single-transition&gt;</span> ]*
            </td>
          </tr>
          <tr>
            <td>
              <em>Initial:</em>
            </td>
            <td>
              see individual properties
            </td>
          </tr>
          <tr>
            <td>
              <em>Applies to:</em>
            </td>
            <td>
              all elements, :before and :after pseudo elements
            </td>
          </tr>
          <tr>
            <td>
              <em>Inherited:</em>
            </td>
            <td>
              no
            </td>
          </tr>
          <tr>
            <td>
              <em>Animatable:</em>
            </td>
            <td>
              no
            </td>
          </tr>
          <tr>
            <td>
              <em>Percentages:</em>
            </td>
            <td>
              N/A
            </td>
          </tr>
          <tr>
            <td>
              <em>Media:</em>
            </td>
            <td>
              interactive
            </td>
          </tr>
          <tr>
            <td>
              <em>Computed value:</em>
            </td>
            <td>
              see individual properties
            </td>
          </tr>
          <tr>
            <td>
              <em>Canonical order:</em>
            </td>
            <td>
              <abbr title="follows order of property value definition">per grammar</abbr>
            </td>
          </tr>
        </tbody>
      </table>

      <div class="prod">
        <dfn id="single-transition">&lt;single-transition&gt;</dfn> = [ none | <span>&lt;single-transition-property&gt;</span> ] || <span>&lt;time&gt;</span> || <span>&lt;single-transition-timing-function&gt;</span> || <span>&lt;time&gt;</span>
      </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 <span>&lt;single-transition&gt;</span> in the shorthand,
        and any of the transitions has
        ''none'' as the <span>&lt;single-transition-property&gt;</span>,
        then the declaration is invalid.
      </p>

      <h2 id="starting">
        Starting of transitions
      </h2>

      <p>
        When the computed value of an animatable property changes,
        implementations must decide what transitions to start based on
        the values of the 'transition-property', 'transition-duration',
        'transition-timing-function', and 'transition-delay' properties
        at the time the animatable property would first have its new
        computed value.
        This means that when one of these 'transition-*' properties
        changes at the same time as
        a property whose change might transition,
        it is the <em>new</em> values of the 'transition-*' properties
        that control the transition.
      </p>
      <div class="example" id="manual-reversing-example">
        <p style="display:none">
          Example(s):
        </p>
        <p>This provides a way for authors to specify different values
        of the 'transition-*' properties for the &ldquo;forward&rdquo;
        and &ldquo;reverse&rdquo; transitions (but see <a
        href="#reversing">below</a> for special reversing behavior when
        an <em>incomplete</em> transition is interrupted).  Authors can
        specify the value of 'transition-duration',
        'transition-timing-function', or 'transition-delay' in the same
        rule where they specify the value that triggers the transition,
        or can change these properties at the same time as they change
        the property that triggers the transition.  Since it's the new
        values of these 'transition-*' properties that affect the
        transition, these values will be used for the transitions
        <em>to</em> the associated transitioning values.  For example:
         </p>
        <pre>li {
  transition: background-color linear 1s;
  background: blue;
}
li:hover {
  background-color: green;
  transition-duration: 2s; /* applies to the transition *to* the :hover state */
}</pre>
        <p>
          When a list item with these style rules enters the :hover
          state, the computed 'transition-duration' at the time that
          'background-color' would have its new value (''green'') is ''2s'',
          so the transition from 'blue' to 'green' takes 2 seconds.
          However, when the list item leaves the :hover state, the
          transition from ''green'' to ''blue'' takes 1 second.
        </p>
      </div>

      <p>
        Various things can cause the computed style of an element to change,
        or for an element to start or stop having computed style.
        (For the purposes of this specification,
        an element has computed style when it is in the document tree,
        and does not have computed style when it is not in the document tree.)
        These include
        insertion and removal of elements from the document tree
        (which both changes whether those elements have computed styles 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 computed styles are updated.
        However,
        when an implementation updates the computed style for an element
        to reflect one of these changes,
        it must update the computed style for all 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>style change event</dfn>.
        (Implementations typically have a <span>style change event</span> to
        correspond with their desired screen refresh rate,
        and when up-to-date computed style is needed
        for a script API that depends on it.)
      </p>

      <p>
        Since this specification does not define
        when a <span>style change event</span> occurs,
        and thus what changes to computed values are considered simultaneous,
        authors should be aware that changing any of the transition
        properties a small amount of time after making a change that
        might transition can result in behavior that varies between
        implementations, since the changes might be considered
        simultaneous in some implementations but not others.
      </p>

      <p>
        When a <span>style change event</span> occurs,
        implementations must start transitions based on
        the computed styles that changed in that event.
        If an element does not have a computed style
        either before or after the style change event,
        then transitions are not started for that element
        in that style change event.
        Otherwise,
        define the <span>before-change style</span> as
        the computed style for the element as of
        the previous <span>style change event</span>,
        except with any styles derived from declarative
        animations such as CSS Transitions, CSS Animations
        ([[CSS3-ANIMATIONS]]),
        and SMIL Animations ([[SMIL-ANIMATION]], [[SVG11]])
        updated to the current time.
        Likewise, define the <span>after-change style</span> as
        the computed style for the element based on the information
        known at the start of that <span>style change event</span>,
        in other words,
        excluding any changes resulting from CSS Transitions
        that start during that <span>style change event</span>.
      </p>

      <div class="note">
        <p>
          Note that this definition of the <span>after-change style</span>
          means that a single change
          can start a transition on the same property
          on both an ancestor element and its descendant element.
          This can happen when a property change is inherited
          from one element with 'transition-*' properties
          that say to animate the changing property
          to another element with 'transition-*' properties
          that also say to animate the changing property.
        </p>

        <p>
          When this happens, both transitions will run,
          and the transition on the descendant will override
          the transition on the ancestor
          because of the normal
          CSS cascading and inheritance rules ([[CSS3CASCADE]]).
        </p>