Overview.bs

<h1>CSS Transitions</h1>

  <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>


<pre class="metadata">
Status: ED
Work Status: Refining
Shortname: css-transitions
Group: csswg
Level: 1
TR: https://www.w3.org/TR/css3-transitions/
Previous version: https://www.w3.org/TR/2013/WD-css3-transitions-20131119/
ED: https://drafts.csswg.org/css-transitions/
Editor: L. David Baron, Mozilla https://www.mozilla.org/, http://dbaron.org/
Editor: Dean Jackson, Apple Inc https://www.apple.com/, dino@apple.com
Editor: David Hyatt, Apple Inc https://www.apple.com/, hyatt@apple.com
Editor: Chris Marrin, Apple Inc https://www.apple.com/, cmarrin@apple.com
Issue Tracking: Bugzilla bugs for this level https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&amp;product=CSS&amp;component=Transitions&amp;resolution=---&amp;status_whiteboard=defer%20to%20level%202&amp;status_whiteboard_type=notregexp
Issue Tracking: Bugzilla bugs for all levels https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&amp;product=CSS&amp;component=Transitions&amp;resolution=---
Abstract: CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.
Status Text: <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>The following behaviors are at risk: <ul><li><a href="#discrete-interpolation-at-risk">Interpolation in steps of property types that cannot be interpolated</a></li></ul>
Ignored Terms: float
Ignored Vars: x1, x2, y1, y2
Link Defaults: css-transforms (property) transform
</pre>
<!-- FIXME: font-size and font-weight link defaults don't work -->
<pre class="link-defaults">
spec:css21; type:property;
    text:top
    text:right
    text:bottom
    text:left
    text:margin-top
    text:margin-right
    text:margin-bottom
    text:margin-left
    text:padding-top
    text:padding-right
    text:padding-bottom
    text:padding-left
    text:border-top-color
    text:border-right-color
    text:border-bottom-color
    text:border-left-color
    text:border-top-width
    text:border-right-width
    text:border-bottom-width
    text:border-left-width
    text:background-color
    text:background-position
    text:border-spacing
    text:width
    text:height
    text:min-width
    text:min-height
    text:max-width
    text:max-height
    text:clip
    text:letter-spacing
    text:line-height
    text:outline-color
    text:outline-width
    text:text-indent
    text:font-size
    text:font-weight
    text:vertical-align
    text:visibility
    text:word-spacing
    text:z-index
spec:css-color-3;
    type:property;
        text:color
        text:opacity
    type:value
        text:green
        text:blue
        text:transparent
spec:css-values-3; type:type; text:<time>
</pre>
<!-- FIXME: These overrides aren't great for dev/TR switching -->
<pre class="anchors">
url: https://drafts.csswg.org/css-backgrounds-3/#shadow-inset; type: value; for: shadow; text: inset;
url: https://www.w3.org/TR/CSS21/visufx.html#propdef-visibility; type: value; for: visibility; text: visible;
urlPrefix: https://drafts.csswg.org/css-color-3/; type: value;
    text: transparent
    text: blue
    text: green
url: http://w3c.github.io/dom/#constructing-events; type: dfn; text: event constructor;
url: https://html.spec.whatwg.org/multipage/infrastructure.html#concept-event-dispatch; type: dfn; text: dispatch;
</pre>
</dl>

Introduction {#introduction}
============================

      <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>

<span id="transitions-">Transitions</span> {#transitions}
=========================================================

      <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 <a>computed value</a> of a property transitions over time from the old value to the new value. Therefore if a script queries the <a>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 '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="https://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>

<span id="the-transition-property-property-">The 'transition-property' Property</span> {#transition-property-property}
----------------------------------------------------------------------------------------------------------------------

      <p>
        The 'transition-property' property specifies the name of the CSS property to which the transition is applied.
      </p>
      <pre class="propdef">
        Name: transition-property
        Value: ''transition-property/none'' | <<single-transition-property>>#
        Initial: ''transition-property/all''
        Applies to: all elements, ::before and ::after pseudo elements
        Inherited: no
        Animatable: no
        Percentages: N/A
        Media: visual
        Computed value: Same as specified value.
        Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
      </pre>

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

      <p>
        A value of
        <dfn value for="transition-property">none</dfn>
        means that no property will transition.
        Otherwise, a list of properties to be transitioned, or the
        keyword <dfn value for="transition-property">all</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 '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 <<custom-ident>> production in <<single-transition-property>>
        also excludes the keyword ''transition-property/none'',
        in addition to the keywords always excluded from <<custom-ident>>.
        This means that
        ''transition-property/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.
      </p>

      <p>
        For the keyword ''transition-property/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
        ''transition-property/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 ''transition-property/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 ''transition-property/all'' value and 'all' shorthand
        property work in similar ways, so the
        ''transition-property/all'' value is just like a shorthand that
        covers all properties.
      </p>

<span id="the-transition-duration-property-">The 'transition-duration' Property</span> {#transition-duration-property}
----------------------------------------------------------------------------------------------------------------------

      <p>
        The 'transition-duration' property defines the length of time that a transition takes.
      </p>
      <pre class="propdef">
        Name: transition-duration
        Value: <<time>>#
        Initial: ''0s''
        Applies to: all elements, ::before and ::after pseudo elements
        Inherited: no
        Animatable: no
        Percentages: N/A
        Media: interactive
        Computed value: Same as specified value.
        Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
      </pre>
      <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>

<span id="transition-timing-function_tag">The 'transition-timing-function' Property</span> {#transition-timing-function-property}
---------------------------------------------------------------------------------------------------------------------------------

      <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>
      <pre class="propdef">
        Name: transition-timing-function
        Value: <<single-transition-timing-function>>#
        Initial: ''transition-timing-function/ease''
        Applies to: all elements, ::before and ::after pseudo elements
        Inherited: no
        Animatable: no
        Percentages: N/A
        Media: interactive
        Computed value: Same as specified value.
        Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
      </pre>
      <div class="prod">
        <dfn type 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'' | <a lt="steps()" function>steps</a>(<<integer>>[, [ ''start'' | ''end'' ] ]?) | <a lt="cubic-bezier()" function>cubic-bezier</a>(<<number>>, <<number>>, <<number>>, <<number>>)
      </div>
      <p>
        The timing functions have the following definitions.
      </p>
      <dl dfn-type="value" dfn-for="transition-timing-function, <single-transition-timing-function>">
        <dt><dfn>ease</dfn></dt>
        <dd>
          The ease function is equivalent to <a lt="cubic-bezier()" function>cubic-bezier(0.25, 0.1, 0.25, 1)</a>.
        </dd>
        <dt><dfn>linear</dfn></dt>
        <dd>
          The linear function is equivalent to <a lt="cubic-bezier()" function>cubic-bezier(0, 0, 1, 1)</a>.
        </dd>
        <dt><dfn>ease-in</dfn></dt>
        <dd>
          The ease-in function is equivalent to <a lt="cubic-bezier()" function>cubic-bezier(0.42, 0, 1, 1)</a>.
        </dd>
        <dt><dfn>ease-out</dfn></dt>
        <dd>
          The ease-out function is equivalent to <a lt="cubic-bezier()" function>cubic-bezier(0, 0, 0.58, 1)</a>.
        </dd>
        <dt><dfn>ease-in-out</dfn></dt>
        <dd>
          The ease-in-out function is equivalent to <a lt="cubic-bezier()" function>cubic-bezier(0.42, 0, 0.58, 1)</a>.
        </dd>
        <dt><dfn>step-start</dfn></dt>
        <dd>
          The step-start function is equivalent to <a lt="steps()" function>steps(1, start)</a>.
        </dd>
        <dt><dfn>step-end</dfn></dt>
        <dd>
          The step-end function is equivalent to <a lt="steps()" function>steps(1, end)</a>.
        </dd>
        <dt><dfn function lt="steps()">steps(<<integer>>[, [ start | end ] ]?)</dfn></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 <dfn value for="steps()">start</dfn> or <dfn value for="steps()">end</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 ''end''.
        </dd>
        <dt><dfn function lt="cubic-bezier()">cubic-bezier(<<number>>, <<number>>, <<number>>, <<number>>)</dfn></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 (<var>x1</var>, <var>y1</var>, <var>x2</var>, <var>y2</var>). Both x values must be
          in the range [0, 1] or the definition is invalid. The y values can
          exceed this range.
        </dd>
      </dl>

<span id="the-transition-delay-property-">The 'transition-delay' Property</span> {#transition-delay-property}
-------------------------------------------------------------------------------------------------------------

      <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>
      <pre class="propdef">
        Name: transition-delay
        Value: <<time>>#
        Initial: ''0s''
        Applies to: all elements, ::before and ::after pseudo elements
        Inherited: no
        Animatable: no
        Percentages: N/A
        Media: interactive
        Computed value: Same as specified value.
        Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
      </pre>

<span id="the-transition-shorthand-property-">The 'transition' Shorthand Property</span> {#transition-shorthand-property}
-------------------------------------------------------------------------------------------------------------------------

      <p>
        The 'transition' shorthand property combines the four properties described above into a single property.
      </p>
      <pre class="propdef">
        Name: transition
        Value: <<single-transition>>#
        Initial: see individual properties
        Applies to: all elements, ::before and ::after pseudo elements
        Inherited: no
        Animatable: no
        Percentages: N/A
        Media: interactive
        Computed value: see individual properties
        Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>
      </pre>

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

Starting of transitions {#starting}
===================================

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

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

      <div class="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, ''transition: 4s text-indent'')
          and a child element that inherits the parent's value specifies
          a transition of the same property for a shorter duration
          (say, ''transition: 1s text-indent'').
          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>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>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>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 [[!CSS3CASCADE]]
        without updating the <a>computed value</a>
        (which means merely that implementations cannot avoid
        meeting requirements of this specification
        by claiming not to have updated the <a>computed value</a>
        as part of handling a style change).
        However,
        when an implementation updates the <a>computed value</a> of a
        property on an element
        to reflect one of these changes,
        or computes the <a>computed value</a> of a property on an element
        newly added to the document,
        it must update the <a>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 export>style change event</dfn>.
        (Implementations typically have a <a>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>style change event</a> 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 <a>style change event</a> occurs,
        implementations must start transitions based on
        the <a>computed values</a> that changed in that event.
        If an element is not in the document during that
        style change even or was not in the document during
        the previous style change event,
        then transitions are not started for that element
        in that style change event.
        Otherwise,
        define the <dfn export>before-change style</dfn> as
        the <a>computed values</a> of all properties on the element as of
        the previous <a>style change event</a>,
        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 <dfn export>after-change style</dfn> as
        the <a>computed values</a> of all properties
        on the element based on the information
        known at the start of that <a>style change event</a>,
        but excluding any styles from CSS Transitions in the computation,
        and inheriting from
        the <a>after-change style</a> of the parent.
      </p>

      <div class="note">
        <p>
          Note that this definition of the <a>after-change style</a>
          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>

        <p>
          If the transition on the descendant completes before
          the transition on the ancestor,
          the descendant will then resume inheriting
          the (still transitioning) value from its parent.
          This effect is likely not a desirable effect,
          but it is essentially doing what the author asked for.
        </p>
      </div>

      <p>
        For each element with a <a>before-change style</a> and
        an <a>after-change style</a>,
        and each property (other than shorthands),
        define the <dfn export>matching transition-property value</dfn> as
        the last value in the
        'transition-property' in the element's <a>after-change style</a>
        that matches the property,
        as described in
        [[#transition-property-property]].
        If there is such a value, then corresponding to it, there is
        a <dfn export>matching transition duration</dfn>,
        a <dfn export>matching transition delay</dfn>, and
        a <dfn export>matching transition timing function</dfn>
        in the values in the <a>after-change style</a> of
        'transition-duration', 'transition-delay', and 'transition-timing-function'
        (see <a href="#list-matching">the rules on matching lists</a>).
        Define the <dfn export for="transition">combined duration</dfn> of the transition
        as the sum of max(<a>matching transition duration</a>, ''0s'') and
        the <a>matching transition delay</a>.
        For each element and property, the implementation must act
        as follows:
      </p>

      <ol>
      <li>
        If all of the following are true:
        <ul>
          <li>
            the element does not have
            a <a>running transition</a> for the property,
          </li>
          <li>
            the <a>before-change style</a> is different from
            and can be interpolated with
            the <a>after-change style</a> for that property,
          </li>
          <li>
            the element does not have a <a>completed transition</a>
            for the property
            or the <a>end value</a> of the <a>completed transition</a>
            is different from the <a>after-change style</a> for the property,
          </li>
          <li>
            there is a <a>matching transition-property value</a>, and
          </li>
          <li>
            the <a>combined duration</a> is greater than ''0s'',
          </li>
        </ul>
        then implementations must
        remove the <a>completed transition</a> (if present) from the set
        of completed transitions and
        start a transition whose:
        <ul>
          <li>
            <a>start time</a> is
            the time of the <a>style change event</a> plus
            the <a>matching transition delay</a>,
          </li>
          <li>
            <a>end time</a> is
            the <a>start time</a> plus
            the <a>matching transition duration</a>,
          </li>
          <li>
            <a>start value</a> is
            the value of the transitioning property
            in the <a>before-change style</a>,
          </li>
          <li>
            <a>end value</a> is
            the value of the transitioning property
            in the <a>after-change style</a>,
          </li>
          <li>
            <a>reversing-adjusted start value</a> is the same as
            the <a>start value</a>, and
          <li>
            <a>reversing shortening factor</a> is 1.
          </li>
        </ul>
      </li>
      <li>
        Otherwise,
        if the element has a <a>completed transition</a> for the property
        and the <a>end value</a> of the <a>completed transition</a>
        is different from the <a>after-change style</a> for the property,
        then implementations must
        remove the <a>completed transition</a> from the set of
        <a>completed transitions</a>.
      </li>
      <li>
        If the element has a <a>running transition</a> or
        <a>completed transition</a> for the property,
        and there is <strong>not</strong>
        a <a>matching transition-property value</a>,
        then implementations must
        <a>cancel</a> the <a>running transition</a>
        or remove the <a>completed transition</a> from the set of
        <a>completed transitions</a>.
      </li>
      <li>
        If the element has a <a>running transition</a> for the property,
        there is a <a>matching transition-property value</a>,
        and the <a>end value</a> of the <a>running transition</a> is
        <strong>not</strong> equal to the value of the property in the
        <a>after-change style</a>, then:
        <ol>
          <li>
            If the <a>current value</a> of the property
            in the <a>running transition</a>
            is equal to
            the value of the property in the <a>after-change style</a>,
            or if these two values cannot be interpolated,
            then implementations must
            <a>cancel</a> the <a>running transition</a>.
          </li>
          <li>
            Otherwise, if the <a>combined duration</a> is
            less than or equal to ''0s'',
            or if the
            <a>current value</a> of the property in the <a>running transition</a>
            cannot be interpolated with
            the value of the property in the <a>after-change style</a>,
            then implementations must
            <a>cancel</a> the <a>running transition</a>.
          </li>
          <li>
            Otherwise, if the <a>reversing-adjusted start value</a>
            of the <a>running transition</a> is the same as the value of
            the property in the <a>after-change style</a>
            <span class="note">(see the
            <a href="#reversing">section on reversing of
            transitions</a> for why these case exists)</span>,
            implementations must
            <a>cancel</a> the <a>running transition</a> and
            start a new transition whose:
            <ul>
              <li>
                <a>reversing-adjusted start value</a> is
                the <a>end value</a> of the
                <a>running transition</a>
                <span class="note">(Note: This represents the logical start state of
                the transition, and allows some calculations to ignore that
                the transition started before that state was reached, which
                in turn allows repeated reversals of the same transition to
                work correctly),</span>
              <li>
                <a>reversing shortening factor</a>
                is the absolute value, clamped to the range [0, 1],
                of the sum of:
                <ol>
                  <li>the output of the timing function of the old transition
                  at the time of the <a>style change event</a>,
                  times the <a>reversing shortening factor</a> of the
                  old transition</li>
                  <li>1 minus the <a>reversing shortening factor</a> of
                  the old transition.</li>
                </ol>
                <span class="note">Note: This represents the portion of the
                space between the <a>reversing-adjusted start value</a>
                and the <a>end value</a> that the old transition has
                traversed (in amounts of the value, not time), except with the
                absolute value and clamping to handle timing functions that
                have y1 or y2 outside the range [0, 1].</span>
              </li>
              <li>
                <a>start time</a> is
                the time of the <a>style change event</a> plus:
                <ol>
                  <li>if the <a>matching transition delay</a>
                      is nonnegative,
                      the <a>matching transition delay</a>, or
                  <li>if the <a>matching transition delay</a>
                      is negative,
                      the product of
                      the new transition's
                      <a>reversing shortening factor</a> and
                      the <a>matching transition delay</a>,
                </ol>
              </li>
              <li>
                <a>end time</a> is
                the <a>start time</a> plus the product of
                the <a>matching transition duration</a> and
                the new transition's <a>reversing shortening factor</a>,
              </li>
              <li>
                <a>start value</a> is
                the <a>current value</a> of the property
                in the <a>running transition</a>,
              </li>
              <li>
                <a>end value</a> is
                the value of the property
                in the <a>after-change style</a>,
              </li>
            </ul>
          </li>
          <li>
            Otherwise, implementations must
            <a>cancel</a> the <a>running transition</a>
            and start a new transition whose:
            <ul>
              <li>
                <a>start time</a> is
                the time of the <a>style change event</a> plus
                the <a>matching transition delay</a>,
              </li>
              <li>
                <a>end time</a> is
                the <a>start time</a> plus
                the <a>matching transition duration</a>,
              </li>
              <li>
                <a>start value</a> is
                the <a>current value</a> of the property
                in the <a>running transition</a>,
              </li>
              <li>
                <a>end value</a> is
                the value of the property
                in the <a>after-change style</a>,
              </li>
              <li>
                <a>reversing-adjusted start value</a> is the same as
                the <a>start value</a>, and
              <li>
                <a>reversing shortening factor</a> is 1.
              </li>
            </ul>
          </li>
        </ol>
      </li>

      </ol>

      <div class="note">
        <p>
          Note that the above rules mean that
          when the computed value of an animatable property changes,
          the transitions that start are 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,
          when the transitions are between two states
          (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>
      </div>

      <p class="note">
        Note that once the transition of a property has started
        (including being in its delay phase),
        it continues running based on
        the original timing function, duration, and
        delay, even if the 'transition-timing-function',
        'transition-duration', or 'transition-delay' property changes
        before the transition is complete.  However, if the
        'transition-property' property changes such that the transition
        would not have started, the transition stops (and the
        property immediately changes to its final value).
      </p>

      <p class="note">
        Note that above rules mean that
        transitions do not start when the computed
        value of a property changes as a result of declarative animation
        (as opposed to scripted animation).
        This happens because the before-change style includes up-to-date
        style for declarative animations.
      </p>

Faster reversing of interrupted transitions {#reversing}
--------------------------------------------------------

      <div class="note">

      <p>
        Many common transitions effects involve transitions between two states,
        such as the transition that occurs when the mouse pointer moves
        over a user interface element, and then later moves out of that element.
        With these effects, it is common for a running transition
        to be interrupted before it completes,
        and the property reset to the starting value of that transition.
        An example is a hover effect on an element,
        where a transition starts when the pointer enters the element,
        and then the pointer exits the element before the effect has completed.
        If the outgoing and incoming transitions
        are executed using their specified durations and timing functions,
        the resulting effect can be distractingly asymmetric
        because the second transition
        takes the full specified time to move a shortened distance.
        Instead, this specification makes second transition shorter.
      </p>

      <p>
        The mechanism the above rules use to cause this involves the
        <a>reversing shortening factor</a> and the
        <a>reversing-adjusted start value</a>.
        In particular, the reversing behavior is present whenever
        the <a>reversing shortening factor</a> is less than 1.
      </p>

      <p class="note">
        Note that these rules do not fully address the problem for
        transition patterns that involve more than two states.
      </p>

      <p class="note">
        Note that these rules lead to the entire timing function of the
        new transition being used, rather than jumping into the middle
        of a timing function, which can create a jarring effect.
      </p>

      <p class="note">
        This was one of several possibilities that was considered by the
        working group.  See the
        <a href="transition-reversing-demo">reversing demo</a>
        demonstrating a number of them, leading to a working group
        resolution made on 2013-06-07 and edits made on 2013-11-11.
      </p>

      </div>

Application of transitions {#application}
=========================================

      <p>
        When a property on an element is undergoing a transition
        (that is, when or after the transition has started and before the
        <a>end time</a> of the transition)
        the transition adds a style called the <dfn export>current value</dfn>
        to the CSS cascade
        at the level defined for CSS Transitions in [[!CSS3CASCADE]].
      </p>

      <p class="note">
        Note that this means that computed values
        resulting from CSS transitions
        can inherit to descendants just like
        any other computed values.
        In the normal case, this means that
        a transition of an inherited property
        applies to descendant elements
        just as an author would expect.
      </p>

      <p>
        Implementations must add this value to the cascade
        if and only if
        that property is not currently
        undergoing a CSS Animation ([[!CSS3-ANIMATIONS]]) on the same element.
      </p>

      <p class="note">
        Note that this behavior of transitions not applying to the cascade
        when an animation on the same element and property is running
        does not affect whether the transition has started or ended.
        APIs that detect whether transitions are running
        (such as <a href="#transition-events">transition events</a>)
        still report that a transition is running.
      </p>

      <p>
        If the current time is at or before the
        <a>start time</a> of the transition
        (that is, during the delay phase of the transition),
        the <a>current value</a> is a specified style that will compute
        to the <a>start value</a> of the transition.
      </p>

      <p>
        If the current time is after the
        <a>start time</a> of the transition
        (that is, during the duration phase of the transition),
        the <a>current value</a> is a specified style that will compute
        to the <a href="#animatable-types">result of interpolating the property</a>
        using the <a>start value</a> of the transition as
        <var>V</var><sub>start</sub>,
        using the <a>end value</a> of the transition as
        <var>V</var><sub>end</sub>,
        and using (current time - start time) / (end time - start time)
        as the input to the timing function.
      </p>

Completion of transitions {#complete}
=====================================

      <p>
        <a>Running transitions</a>
        <dfn export for="transition" id="dfn-complete">complete</dfn>
        at a time that equal to or after their end time,
        but prior to to the first <a>style change event</a>
        whose time is equal to or after their <a>end time</a>.
        When a transition completes,
        implementations must move
        all transitions that complete at that time
        from the set of <a>running transitions</a>
        to the set of <a>completed transitions</a>
        and then fire the <a href="#transition-events">events</a>
        for those completions.
        <span class="note">(Note that doing otherwise, that is,
        firing some of the events before doing all of the moving
        from <a>running transitions</a> to <a>completed transitions</a>,
        could allow
        a style change event to happen
        without the necessary transitions completing,
        since firing the event could cause a style change event,
        if an event handler requests up-to-date computed style or
        layout data.)</span>
      </p>

<span id="transition-events-">Transition Events</span> {#transition-events}
===========================================================================
      <p>
        The completion of a CSS Transition generates a corresponding <a href="https://www.w3.org/TR/DOM-Level-2-Events/events.html">DOM Event</a>.
        An event is <a>dispatched</a> to the element
        for each property that undergoes a transition on that element.
        This allows a content developer to perform actions that synchronize
        with the completion of a transition.
      </p>
      <p>
        Each event provides the name of the property the transition is
        associated with as well as the duration of the transition.
      </p>
      <dl>
        <dt>
          <b>Interface <dfn interface id="Events-TransitionEvent">TransitionEvent</dfn></b>
        </dt>
        <dd>
          <p>
            The {{TransitionEvent}} interface provides specific contextual information associated with transitions.
          </p>
          <dl>
            <dt>
              <b>IDL Definition</b>
            </dt>
            <dd>
              <div class='idl-code'>
                <pre class='idl'>
  [Constructor(DOMString type, optional TransitionEventInit transitionEventInitDict)]
  interface TransitionEvent : Event {
    readonly attribute DOMString          propertyName;
    readonly attribute float              elapsedTime;
    readonly attribute DOMString          pseudoElement;
  };

  dictionary TransitionEventInit : EventInit {
    DOMString propertyName = "";
    float elapsedTime = 0.0;
    DOMString pseudoElement = "";
  };
  </pre>
              </div>
            </dd>
            <dt>
              <b>Attributes</b>
            </dt>
            <dd>
              <dl>
                <dt>
                  <code class='attribute-name'><dfn attribute for="TransitionEvent" id="Events-TransitionEvent-propertyName">propertyName</dfn></code> of type <code>DOMString</code>, readonly
                </dt>
                <dd>
                  The name of the CSS property associated with the transition.
                </dd>
              </dl>
              <dl>
                <dt>
                  <code class='attribute-name'><dfn attribute for="TransitionEvent" id="Events-TransitionEvent-elapsedTime">elapsedTime</dfn></code> of type <code>float</code>, readonly
                </dt>
                <dd>
                  The amount of time the transition has been running, in seconds, when this event fired. Note that this value is not affected by the value of <code class="property">transition-delay</code>.
                </dd>
              </dl>
              <dl>
                <dt>
                  <code class='attribute-name'><dfn attribute for="TransitionEvent" id="Events-TransitionEvent-pseudoElement">pseudoElement</dfn></code> of type <code>DOMString</code>, readonly
                </dt>
                <dd>
                  The name (beginning with two colons) of the CSS
                  pseudo-element on which the transition occurred (in
                  which case the target of the event is that
                  pseudo-element's corresponding element), or the empty
                  string if the transition occurred on an element (which
                  means the target of the event is that element).
                </dd>
              </dl>
            </dd>
          </dl>
          <p>
            <code id="TransitionEvent-constructor">TransitionEvent(type, transitionEventInitDict)</code>
            is an <a>event constructor</a>.
          </p>
        </dd>
      </dl>
      <p>
        There is one type of transition event available.
      </p>
      <dl>
        <dt>
          <b><dfn event for="Element" id="transitionend">transitionend</dfn></b>
        </dt>
        <dd>
          The {{transitionend}} event occurs at the completion of the transition. In the
          case where a transition is removed before completion, such as if the
          transition-property is removed, then the event will not fire.
          <ul>
            <li>Bubbles: Yes
            </li>
            <li>Cancelable: No
            </li>
            <li>Context Info: propertyName, elapsedTime, pseudoElement
            </li>
          </ul>
        </dd>
      </dl>

<span id="animation-of-property-types-">Animation of property types</span> {#animatable-types}
==============================================================================================

      <p>
        Some property types can be interpolated,
        which means they can animate smoothly from one value to another.
        Other property types cannot, and thus animate only in a single
        step from one value to the other.
      </p>

Animation of interpolated property types {#interpolated-types}
--------------------------------------------------------------

      <p>
        When interpolating between two values,
        <var>V</var><sub>start</sub> and <var>V</var><sub>end</sub>,
        interpolation is done using the output <var>p</var> of the timing function,
        which gives the portion of the value space
        that the interpolation has crossed.
        Thus the result of the interpolation is
        <var>V</var><sub>res</sub> =
          (1 - <var>p</var>) &sdot; <var>V</var><sub>start</sub> +
          <var>p</var> &sdot; <var>V</var><sub>end</sub>.
      </p>

      <p>
        However, if this value (<var>V</var><sub>res</sub>)
        is outside the allowed range of values for the property,
        then it is clamped to that range.
        This can occur if <var>p</var> is outside of the range 0 to 1,
        which can occur if a timing function is specified
        with a <var>y1</var> or <var>y2</var> that is outside the range 0 to 1.
      </p>

      <p>
        The following describes how each property type undergoes transition or
        animation.
      </p>

      <ul>
        <li id="animtype-color">
          <strong>color</strong>: interpolated via red, green, blue and alpha
          components (treating each as a number, see below).
          The interpolation is done between premultiplied colors
          (that is, colors for which the red, green, and blue components
          specified have been multiplied by the alpha).
        </li>
        <li id="animtype-length">
          <strong>length</strong>: interpolated as real numbers.
        </li>
        <li id="animtype-percentage">
          <strong>percentage</strong>: interpolated as real numbers.
        </li>
        <li id="animtype-lpcalc">
          <strong>length, percentage, or calc</strong>: when both values
          are lengths, interpolated as lengths; when both values are
          percentages, interpolated as percentages; otherwise, both
          values are converted into a ''calc()'' function that is the
          sum of a length and a percentage (each possibly zero), and
          these ''calc()'' functions have each half interpolated as real
          numbers.
        </li>
        <li id="animtype-integer">
          <strong>integer</strong>: interpolated via discrete steps (whole
          numbers). The interpolation happens in real number space and is
          converted to an integer by rounding to the nearest integer, with
          values halfway between a pair of integers rounded towards
          positive infinity.
        </li>
        <li id="animtype-font-weight">
          <strong>font weight</strong>: interpolated via discrete steps
          (multiples of 100). The interpolation happens in real number
          space and is converted to an integer by rounding to the
          nearest multiple of 100, with values halfway between multiples
          of 100 rounded towards positive infinity.
        </li>
        <li id="animtype-number">
          <strong>number</strong>: interpolated as real (floating point)
          numbers.
        </li>
        <li id="animtype-rect">
          <strong>rectangle</strong>: interpolated via the x, y,
          width and height components (treating each as a number).
        </li>
        <li id="animtype-visibility">
          <strong>visibility</strong>: if one of the values is
          ''visibility/visible'', interpolated as a discrete step where values of the
          timing function between 0 and 1 map to ''visibility/visible'' and other
          values of the timing function (which occur only at the
          start/end of the transition or as a result of ''cubic-bezier()''
          functions with Y values outside of [0, 1]) map to the closer
          endpoint; if neither value is ''visibility/visible'' then not interpolable.
        </li>
        <li id="animtype-shadow-list">
          <strong>shadow list</strong>: Each shadow in the list
          (treating ''shadow/none'' as a 0-length list)
          is interpolated via the
          color (as <a href="#animtype-color">color</a>) component,
          and x, y, blur, and (when appropriate) spread
          (as <a href="#animtype-length">length</a>) components.
          For each shadow, if both input shadows are ''shadow/inset''
          or both input shadows are not ''shadow/inset'',
          then the interpolated shadow must match the input shadows in that regard.
          If any pair of input shadows has one ''shadow/inset'' and the other not ''shadow/inset'',
          the entire <a href="#animtype-shadow-list">shadow-list</a> is uninterpolable.
          If the lists of shadows have different lengths,
          then the shorter list is padded at the end
          with shadows whose color is ''transparent'',
          all lengths are ''0'',
          and whose ''shadow/inset'' (or not) matches the longer list.
        </li>
        <li id="animtype-gradient">
          <strong>gradient</strong>: interpolated via the
          positions and colors of each stop. They must have the same type
          (radial or linear) and same number of stops in order to be animated.
          <span class="note">Note: [[CSS3-IMAGES]] may extend this
          definition.</span>
        </li>
        <li id="animtype-paintserver">
          <strong>paint server</strong> (SVG): interpolation is only supported
          between: gradient to gradient and color to color. They then
          work as above.
        </li>
        <li id="animtype-simple-list">
          <strong>simple list</strong> of other types:
          If the lists have the same number of items,
          and each pair of values can be interpolated,
          each item in the list is interpolated using
          the rules given for those types.
          Otherwise the values are not interpolable.
        </li>
        <li id="animtype-repeatable-list">
          <strong>repeatable list</strong> of other types:
          The result list has a length that is the least common multiple
          of the lengths of the input lists.
          Each item in the result is the interpolation of the value
          from each input list repeated to the length of the result list.
          If a pair of values cannot be interpolated, then the lists
          are not interpolable.
          <span class="note">
            The repeatable list concept ensures that a list that is
            conceptually repeated to a certain length (as
            'background-origin' is repeated to the length of the
            'background-image' list) or repeated infinitely will
            smoothly transition between any values, and so that the
            computed value will properly represent the result (and
            potentially be inherited correctly).
          </span>
        </li>
      </ul>

      <p>Future specifications may define additional types that can
      be animated.</p>

      <p>See the definition of 'transition-property' for how animation
      of shorthand properties and the ''all'' value is applied to any
      properties (in the shorthand) that can be animated.</p>

Animation in steps of other property types {#step-types}
--------------------------------------------------------

      <p>
        When interpolating between two values that cannot be interpolated,
        <var>V</var><sub>start</sub> and <var>V</var><sub>end</sub>,
        interpolation is done using the output <var>p</var> of the timing function.
        If <var>p</var> is less than 0.5, then the
        result of the interpolation is
        <var>V</var><sub>start</sub>;
        if <var>p</var> is greater than or equal to 0.5, then the
        result of the interpolation is
        <var>V</var><sub>end</sub>.
      </p>

      <p class="note" id="discrete-interpolation-at-risk">
        This is a recent change to which implementations have
        not yet updated.  (Prior to the change CSS Transitions
        and CSS Animations did not run on such changes.)  It's
        possible that it won't be compatible with existing Web content.
        If that is the case, the problem may be mitigated by restricting
        this behavior only to CSS Animations (and not to CSS Transitions),
        and/or restricting it to step timing functions.
      </p>

<span id="animatable-properties-">Animatable properties</span> {#animatable-properties}
=======================================================================================

      <!--
      As resolved in
      http://lists.w3.org/Archives/Public/www-style/2011Sep/0497.html
      -->

      <p>The definition of each CSS property defines
      when the values of that property can be interpolated
      by referring to the definitions of property types
      in the <a href="#animatable-types">previous section</a>.
      The animated value is interpolated from the from and to values when
      both the from and the to values of the property have the type described.
      (When a composite type such as "length, percentage, or calc" is listed,
      this means that both values must fit into that composite type.)
      When multiple types are listed in the form "either A or B",
      both values must be of the same type to be interpolable.</p>

      <p>Otherwise, since the from and to values cannot be interpolated,
      the animation is done <a href="#step-types">in a single step</a>.</p>

      <p>The 'transition-*' properties defined in this specification do
      not undergo transitions.</p>

      <p>For properties that exist at the time this specification was
      developed, this specification defines how they are
      animated.  However, future CSS specifications may define
      additional properties, additional values for existing properties,
      or additional animation behavior of existing values.  In order to
      describe new animation behaviors and to have the definition of
      animation behavior in a more appropriate location, future CSS
      specifications should include an "Animatable:" line in the summary
      of the property's definition (in addition to the other lines
      described in [[CSS21]], <a
      href="https://www.w3.org/TR/CSS21/about.html#property-defs">section
      1.4.2</a>).  This line should say "no" to indicate that a property
      cannot be animated or should reference an animation behavior
      (which may be one of the behaviors in the <a
      href="#animation-of-property-types-">Animation of property
      types</a> section above, or may be a new behavior) to define how
      the property animates.  Such definitions override those given in
      this specification.</p>

      <p class="issue">
        It no longer makes sense for this line to be called
        "Animatable".  It should probably be renamed to "Interpolation",
        and the "no" value renamed to "discrete" or "in steps".
      </p>

<span id="properties-from-css-">Properties from CSS</span> {#animatable-css}
----------------------------------------------------------------------------

      <p>
      The following definitions define the animation behavior for
      properties in CSS Level 2 Revision 1 ([[CSS21]]) and in Level 3 of
      the CSS Color Module ([[CSS3COLOR]]).
      </p>

     <table class="animatable-properties">
       <tr>
         <th>Property Name</th>
         <th>Type</th>
       </tr>
       <tr>
         <td>'background-color'</td><td>as <a href="#animtype-color">color</a></tr>
       <tr>
         <td>'background-position'</td><td>as <a href="#animtype-repeatable-list">repeatable list</a> of <a href="#animtype-simple-list">simple list</a> of <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'border-bottom-color'</td><td>as <a href="#animtype-color">color</a></td>
       </tr>
       <tr>
         <td>'border-bottom-width'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'border-left-color'</td><td>as <a href="#animtype-color">color</a></td>
       </tr>
       <tr>
         <td>'border-left-width'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'border-right-color'</td><td>as <a href="#animtype-color">color</a></td>
       </tr>
       <tr>
         <td>'border-right-width'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'border-spacing'</td><td>as <a href="#animtype-simple-list">simple list</a> of <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'border-top-color'</td><td>as <a href="#animtype-color">color</a></td>
       </tr>
       <tr>
         <td>'border-top-width'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'bottom'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'clip'</td><td>as <a href="#animtype-rect">rectangle</a></td>
       </tr>
       <tr>
         <td>'color'</td><td>as <a href="#animtype-color">color</a></td>
       </tr>
       <tr>
         <td>'font-size'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'font-weight!!property'</td><td>as <a href="#animtype-font-weight">font weight</a></td>
       </tr>
       <tr>
         <td>'height'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'left'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'letter-spacing'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'line-height'</td><td>as either <a href="#animtype-number">number</a> or <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'margin-bottom'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'margin-left'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'margin-right'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'margin-top'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'max-height'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'max-width'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'min-height'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'min-width'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'opacity'</td><td>as <a href="#animtype-number">number</a></td>
       </tr>
       <tr>
         <td>'outline-color'</td><td>as <a href="#animtype-color">color</a></td>
       </tr>
       <tr>
         <td>'outline-width'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'padding-bottom'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'padding-left'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'padding-right'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'padding-top'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'right'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'text-indent'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'text-shadow'</td><td>as <a href="#animtype-shadow-list">shadow list</a></td>
       </tr>
       <tr>
         <td>'top'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'vertical-align'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'visibility'</td><td>as <a href="#animtype-visibility">visibility</a></td>
       </tr>
       <tr>
         <td>'width'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>
       </tr>
       <tr>
         <td>'word-spacing'</td><td>as <a href="#animtype-length">length</a></td>
       </tr>
       <tr>
         <td>'z-index'</td><td>as <a href="#animtype-integer">integer</a></td>
       </tr>
     </table>

<span id="properties-from-svg-">Properties from SVG</span> {#animatable-svg}
----------------------------------------------------------------------------

     <p>
       All properties defined as animatable in the SVG specification, provided
       they are one of the property types listed above.
      </p>

     <!-- <table>
       <tr>
         <th>Property Name</th><th>Type</th>
       </tr>
       <tr>
         <td>stop-color</td><td>as <a href="#animtype-color">color</a></td>
       </tr>
       <tr>
         <td>stop-opacity</td><td>as <a href="#animtype-number">number</a></td>
       </tr>
       <tr>
         <td>fill</td><td>as <a href="#animtype-paintserver">paint server</a></td>
       </tr>
       <tr>
         <td>fill-opacity</td><td>as <a href="#animtype-number">number</a></td>
       </tr>
       <tr>
         <td>stroke</td><td>as <a href="#animtype-paintserver">paint server</a></td>
       </tr>
       <tr>
         <td>stroke-dasharray</td><td>as <a href="#animtype-repeatable-list">repeatable list</a> of <a href="#animtype-number">number</a></td>
       </tr>
       <tr>
         <td>stroke-dashoffset</td><td>as <a href="#animtype-number">number</a></td>
       </tr>
       <tr>
         <td>stroke-miterlimit</td><td>as <a href="#animtype-number">number</a></td>
       </tr>
       <tr>
         <td>stroke-opacity</td><td>as <a href="#animtype-number">number</a></td>
       </tr>
       <tr>
         <td>stroke-width</td><td>as <a href="#animtype-number">number</a></td>
       </tr>
       <tr>
         <td>viewport-fill</td><td>as <a href="#animtype-color">color</a></td>
       </tr>
       <tr>
         <td>viewport-fill-opacity</td><td>as <a href="#animtype-color">color</a></td>
       </tr>
      </table> -->

Security Considerations {#security}
===================================

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

The security implications of this specification are limited
because it doesn't allow Web content to do things
that it could not do before.
Rather, it allows things that could previously be done with script
to be done declaratively,
and it ways that implementations can optimize (for frame rate and
CPU usage).

One of the major categories of optimizations
that implementations can make is implementing animation
of certain high-value properties (such as 'transform' and 'opacity')
run on a browser's compositor thread or process
without updating style or layout on the main Web content thread
unless up-to-date style data are needed.
This optimization often requires allocations of graphics memory
to display the contents of the element being animated.
Implementations should take care to ensure that Web content
cannot trigger unsafe out-of-memory handling
by using large numbers of animations
or animations on elements covering large areas
(where large may be defined in terms of pre-transform or post-transform size).

Privacy Considerations {#privacy}
=================================

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

As for security, the privacy considerations of this specification are limited
because it does not allow Web content to do things that it could not do before.

This specification may provide additional mechanisms that help to determine
characteristics of the user's hardware or software.
However, ability to determine performance characteristics of the user's
hardware or software is common to many Web technologies,
and this specification does not introduce new capabilities.

As described in [[#accessibility]],
implementations may provide mitigations to help users with disabilities.
These mitigations are likely to be detectable by Web content,
which means that users who would benefit from these mitigations
may face a tradeoff between keeping their disability private from
the Web content or benefiting from the mitigation.

Accessibility Considerations {#accessibility}
=============================================

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

Motion {#accessibility-motion}
------------------------------

This specification provides declarative mechanisms for animations
that previously needed to be done using script.
Providing a declarative mechanism has multiple effects:
it makes such animations easier to make and thus likely to be more common,
but it also makes it easier for user agents to modify those animations
if such modifications are needed to meet a user's accessibility needs.

Thus, users who are sensitive to movement,
or who require additional time to read or understand content,
may benefit from user agent features that allow
animations to be disabled or slowed down.
(But see [[#privacy]] for information on the privacy implications
of such mitigations.)

User agent implementors should be aware that Web content
may depend on the firing of <a href="#transition-events">transition events</a>,
so implementations of such mitigations may wish to fire transition events
even if the transitions were not run as continuous animations.
However, it is probably poor practice for Web content to depend on
such events to function correctly.

Cascade {#accessibility-cascade}
--------------------------------

The CSS <a>cascade</a> is a general mechanism in CSS
that allows user needs to interact with author styles.
This specification interacts with the cascade,
but since it only allows animation between values
that result from the existing cascade rules,
it does not interfere with the user's ability to force
CSS properties to have particular values.

The cascade also allows users to disable transitions entirely
by overriding the transition properties.


Changes since Working Draft of 19 November 2013 {#changes}
==========================================================

<p>The following are the substantive changes made since the
<a href="https://www.w3.org/TR/2013/WD-css3-transitions-20131119/">Working Draft
dated 19 November 2013</a>:</p>

<ul>
  <li>Values that cannot be interpolated are transitioned when the timing function crosses its midpoint, instead of not running transitions and changing immediately.</li>
  <li>Canceling and interrupting of running transitions is defined much more precisely.  This includes the after-change style no longer including styles from CSS Transitions.</li>
  <li>Completion of transitions is defined somewhat more precisely.</li>
  <li>The transitionend event is no longer cancelable.  This is since it has no default action, so canceling it would have no meaning.  It also matches the animation events.</li>
  <li>The interpolation of ''shadow/inset'' values on shadow lists is no longer backwards.</li>
  <li>A [[#conformance]] section, [[#security]] section, [[#privacy]] section, [[#accessibility]] section, and [[#idl-index]] have been added</li>
  <li>The identifiers accepted by 'transition-property' are defined in terms of <<custom-ident>>.</li>
  <li>Define a little bit more about when changes to computed values happen, by saying at least that implementations must not update the effects of computed values without actually updating computed values.</li>
</ul>

<p>For more details on these changes, see the version control <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.bs">change log since 2015 January 26</a> and the <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.src.html">change log from 2013 March 28 to 2015 January 26</a>.</p>

<p>For changes in earlier working drafts:</p>

<ol>
  <li>see the <a href="https://www.w3.org/TR/2013/WD-css3-transitions-20131119/#changes">changes section in the 19 November 2013 Working Draft</a>
  <li>see the <a href="https://www.w3.org/TR/2013/WD-css3-transitions-20130212/ChangeLog">the ChangeLog</a> for changes in previous working drafts
  <li>For more details on these changes, see the version control change logs, which are split in three parts because of file renaming: <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.bs">change log since 2015 January 26</a>, <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.src.html">change log from 2013 March 28 to 2015 January 26</a>, <a href="https://hg.csswg.org/drafts/log/tip/css3-transitions/Overview.src.html">change log before 2013 March 28</a>.
</ol>

Acknowledgments {#acknowledgments}
==================================

<p>Thanks especially to the feedback from
Tab Atkins,
Carine Bournez,
Aryeh Gregor,
Vincent Hardy,
Anne van Kesteren,
Cameron McCormack,
Alex Mogilevsky,
Jasper St. Pierre,
Estelle Weyl,
and all the rest of the
<a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.</p>