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<pre class="metadata">
Title: CSS Transitions
Status: ED
Work Status: Refining
Shortname: css-transitions
Group: csswg
Level: 1
TR: https://www.w3.org/TR/css-transitions-1/
Previous version: https://www.w3.org/TR/2018/WD-css-transitions-1-20181011/
Previous version: https://www.w3.org/TR/2017/WD-css-transitions-1-20171130/
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/, https://dbaron.org/, w3cid 15393
Editor: Dean Jackson, Apple Inc https://www.apple.com/, dino@apple.com, w3cid 42080
Editor: Brian Birtles, Mozilla https://www.mozilla.org/, bbirtles@mozilla.com, w3cid 43194
Former Editor: David Hyatt, Apple Inc https://www.apple.com/, hyatt@apple.com, w3cid 34140
Former 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.
Ignored Vars: x1, x2, y1, y2
Link Defaults: css-transforms (property) transform
</pre>
<pre class="link-defaults">
spec:css2; 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-backgrounds-3; type:property;
    text:background-image
    text:background-origin
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://www.w3.org/TR/css3-background/#shadow-inset; type: value; for: shadow; text: inset;
url: https://www.w3.org/TR/css3-background/#box-shadow-none; type: value; for: shadow; text: none;
url: https://www.w3.org/TR/CSS21/visufx.html#propdef-visibility; type: value; for: visibility; text: visible;
urlPrefix: https://www.w3.org/TR/css3-color/; type: value;
    text: transparent
    text: blue
    text: green
url: http://w3c.github.io/dom/#constructing-events; type: dfn; text: event constructor;
urlPrefix: https://html.spec.whatwg.org/multipage/webappapis.html; type: dfn; spec: html
    text: event handlers
    text: event handler event type
    text: event handler content attributes
    text: event handler IDL attributes
urlPrefix: https://html.spec.whatwg.org/multipage/infrastructure.html; type: dfn; spec: html
    text: HTML elements
    text: dispatch; url: concept-event-dispatch
url: https://html.spec.whatwg.org/#document; type: interface; text: Document; spec: html
</pre>


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>
      <figure>
        <img src="transition-example.svg" width="518"
          alt="Example showing the initial, intermediate, and final states of a box whose color and position is interpolated">
        <figcaption>
          Transitions of 'left' and 'background-color'.
        </figcaption>
      </figure>
      <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>
        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 user agent 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: none | <<single-transition-property>>#
        Initial: all
        Applies to: all elements
        Inherited: no
        Percentages: N/A
        Computed value: the keyword ''transition-property/none'' else a list of identifiers
        Animation type: not animatable
      </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, 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 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
        all its longhand sub-properties (or, for
        ''transition-property/all'', all 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
        Inherited: no
        Percentages: N/A
        Computed value: list, each item a duration
        Animation type: not animatable
      </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.
      </p>
      <p>
        Timing functions are defined in the separate CSS Easing Functions module
        [[!css-easing-1]].
        The <a spec=css-easing>input progress value</a> used is the percentage
        of the transition duration, and the <a spec=css-easing>output progress
        value</a> is used as the <var>p</var> value when [=interpolating=]
        the property value (see [[#application]]).
      </p>
      <pre class="propdef">
        Name: transition-timing-function
        Value: <<timing-function>>#
        Initial: ''transition-timing-function/ease''
        Applies to: all elements
        Inherited: no
        Percentages: N/A
        Computed value: as specified
        Animation type: not animatable
      </pre>


<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
        Inherited: no
        Percentages: N/A
        Computed value: list, each item a duration
        Animation type: not animatable
      </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 shorthand">
        Name: transition
        Value: <<single-transition>>#
        Applies to: all elements
        Inherited: no
        Animatable: no
        Percentages: N/A
        Animation type: not animatable
      </pre>

      <div class="prod">
        <dfn type id="single-transition">&lt;single-transition&gt;</dfn> = [ ''transition-property/none'' | <<single-transition-property>> ] || <<time>> || <<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
        ''transition-property/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 <a>cancel</a> any <a>running transitions</a>
        on it and remove transitions on it from 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 event 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 using the computed values of the 'animation-*' properties from the
        <a>before-change style</a>,
        excluding any styles from CSS Transitions in the computation,
        and inheriting from
        the <a>after-change style</a> of the parent.
        Note that this means the <a>after-change style</a> does not differ from
        the <a>before-change style</a> due to newly created or canceled CSS
        Animations.
      </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>.
      </p>

      When comparing the [=before-change style=] and [=after-change style=]
      for a given property,
      the property values are <dfn>transitionable</dfn>
      if they have an [=animation type=] that is
      <em>neither</em> [=not animatable=] <em>nor</em> [=discrete=].

      Note: Even if a <em>property</em> is defined to have an [=animation type=]
      that is <em>not</em> [=discrete=],
      for a particular pair of <em>property values</em>
      the [=animation type=] may be [=discrete=].
      For example,
      the [=animation type=] of the 'box-shadow' property is
      <a lt="combining shadow lists">shadow list</a>,
      which defines that
      when the ''shadow/inset'' keyword is absent in one value
      but present in the other,
      [=discrete=] animation is used.
      As a result
      ''0px 0px black'' and ''inset 10px 10px black''
      are <em>not</em> [=transitionable=].

      For each element and property, the implementation must act
      as follows:

      <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
            the <a>after-change style</a> for that property,
            and the values for the property are [=transitionable=],
          </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 are not [=transitionable=],
            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>
            is not [=transitionable=] 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 expose 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 result of [=interpolating=] the property
        using the following values:

        *   <var ignore>V<sub>a</sub></var>:
            <a>start value</a> of the transition
        *   <var ignore>V<sub>b</sub></var>:
            <a>end value</a> of the transition
        *   <var>p</var>: the output of the
            <a href="#transition-timing-function-property">timing function</a>
            for input (current time - start time) / (end time - start time)
      </p>

      The specific [=interpolation=] procedure to be used
      is defined by the property's [=animation type=].


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

      <p>
        <a>Running transitions</a>
        <dfn export for="transition" id="dfn-complete">complete</dfn>
        at a time that is 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}
===========================================================================

The creation, beginning, completion, and cancellation of CSS transitions
generate corresponding DOM Events.
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 changes to transitions.

Each event provides the name of the property the transition is
associated with as well as the duration of the transition.

## Interface {{TransitionEvent}} ## {#interface-transitionevent}

The {{TransitionEvent}} interface provides specific contextual information
associated with transitions.

### IDL Definition ### {#interface-transitionevent-idl}

<pre class="idl">
  [Exposed=Window,
   Constructor(CSSOMString type, optional TransitionEventInit transitionEventInitDict)]
  interface TransitionEvent : Event {
    readonly attribute CSSOMString propertyName;
    readonly attribute double elapsedTime;
    readonly attribute CSSOMString pseudoElement;
  };

  dictionary TransitionEventInit : EventInit {
    CSSOMString propertyName = "";
    double elapsedTime = 0.0;
    CSSOMString pseudoElement = "";
  };
</pre>

### Attributes ### {#interface-transitionevent-attributes}

:   <code class='attribute-name'><dfn attribute for="TransitionEvent" id="Events-TransitionEvent-propertyName">propertyName</dfn></code>
::  The name of the CSS property associated with the transition.
    <p class="note">Note:  This is always the name of a longhand property.  See 'transition-property' for how specifying shorthand properties causes transitions on longhands.</p>
:   <code class='attribute-name'><dfn attribute for="TransitionEvent" id="Events-TransitionEvent-elapsedTime">elapsedTime</dfn></code>
::  The amount of time the transition has been running, in seconds, when this
    event fired not including any time spent in the delay phase.
    The calculation for of this member is defined along with each event type.
:   <code class='attribute-name'><dfn attribute for="TransitionEvent" id="Events-TransitionEvent-pseudoElement">pseudoElement</dfn></code>
::  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).


<code id="TransitionEvent-constructor"><dfn constructor
for="TransitionEvent">TransitionEvent(type, transitionEventInitDict)</dfn></code>
is an <a>event constructor</a>.


Types of <code>TransitionEvent</code> {#event-transitionevent}
--------------------------------------------------------------

The different types of transition events that can occur are:

  <dl dfn-type=event dfn-for=transitionevent>
    <dt><dfn id=transitionrun>transitionrun</dfn></dt>
    <dd>
      The {{transitionrun}} event occurs when a transition is created (i.e.,
      when it is added to the set of <a>running transitions</a>).

      A negative 'transition-delay' will cause the event to fire with
      an {{TransitionEvent/elapsedTime}} equal to the absolute value of the
      delay capped to the 'transition-duration' of the animation. That is,
      the elapsed time is equal to
      <code>min(max(-'transition-delay', 0), 'transition-duration'</a>)</code>.
      <ul>
        <li>Bubbles: Yes</li>
        <li>Cancelable: No</li>
        <li>Context Info: propertyName, elapsedTime, pseudoElement</li>
      </ul>
    </dd>

    <dt><dfn id=transitionstart>transitionstart</dfn></dt>
    <dd>
      The {{transitionstart}} event occurs when a transition's delay phase ends.

      The value of {{TransitionEvent/elapsedTime}} for {{transitionstart}}
      events is the same as the value used for {{transitionrun}} events.
      <ul>
        <li>Bubbles: Yes</li>
        <li>Cancelable: No</li>
        <li>Context Info: propertyName, elapsedTime, pseudoElement</li>
      </ul>
    </dd>

    <dt><dfn id=transitionend>transitionend</dfn></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.

      The value of {{TransitionEvent/elapsedTime}} for this event is equal to
      the value of 'transition-duration'.
      <ul>
        <li>Bubbles: Yes</li>
        <li>Cancelable: No</li>
        <li>Context Info: propertyName, elapsedTime, pseudoElement</li>
      </ul>
    </dd>

    <dt><dfn id=transitioncancel>transitioncancel</dfn></dt>
    <dd>
      The {{transitioncancel}} event occurs when a transition is <a
      lt="cancel">canceled</a>.

      The {{TransitionEvent/elapsedTime}} for {{transitioncancel}} events is
      the number of seconds from the end of the transition's delay to the
      moment when the transition was canceled.
      If the transition had a negative 'transition-delay', the beginning of the
      transition is the moment equal to the absolute value of 'transition-delay'
      seconds <em>prior</em> to when the transition was actually triggered.
      Alternatively, if the transition had a positive 'transition-delay' and the
      event is fired before the transition's delay has expired, the
      {{AnimationEvent/elapsedTime}} will be zero.
      <ul>
        <li>Bubbles: Yes</li>
        <li>Cancelable: No</li>
        <li>Context Info: propertyName, elapsedTime, pseudoElement</li>
      </ul>
    </dd>
  </dl>

Event handlers on elements, <code>Document</code> objects, and <code>Window</code> objects {#event-handlers-on-elements-document-objects-and-window-objects}
---------------------------------------------------------------------------

The following are the <a>event handlers</a> (and their corresponding <a>event
handler event types</a>) that must be supported by all <a>HTML elements</a>, as
both <a>event handler content attributes</a> and <a>event handler IDL
attributes</a>; and that must be supported by all {{Document}} and {{Window}}
objects, as <a>event handler IDL attributes</a>:

<table class="event-handlers">
  <tr>
    <th><a>Event handler</a></th>
    <th><a>Event handler event type</a></th>
  </tr>
  <tr>
    <td>{{GlobalEventHandlers/ontransitionrun}}</td>
    <td><a idl>transitionrun</a></td>
  </tr>
  <tr>
    <td>{{GlobalEventHandlers/ontransitionstart}}</td>
    <td><a idl>transitionstart</a></td>
  </tr>
  <tr>
    <td>{{GlobalEventHandlers/ontransitionend}}</td>
    <td><a idl>transitionend</a></td>
  </tr>
  <tr>
    <td>{{GlobalEventHandlers/ontransitioncancel}}</td>
    <td><a idl>transitioncancel</a></td>
  </tr>
</table>


DOM Interfaces {#interface-dom}
===============================

This specification extends the {{GlobalEventHandlers}} interface mixin from HTML to
add <a>event handler IDL attributes</a> for <a
href="#transition-events">transition events</a> as defined in [[#event-handlers-on-elements-document-objects-and-window-objects]].

IDL Definition {#interface-globaleventhandlers-idl}
--------------

<pre class="idl">
partial interface mixin GlobalEventHandlers {
  attribute EventHandler ontransitionrun;
  attribute EventHandler ontransitionstart;
  attribute EventHandler ontransitionend;
  attribute EventHandler ontransitioncancel;
};
</pre>

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 30 November 2017 {#changes}
==========================================================

The following are the substantive changes made since the
<a href="https://www.w3.org/TR/2017/WD-css-transitions-1-20171130/">Working Draft
dated 11 October 2018</a>:

*   (Nothing yet)

For more details on these changes, see the version control
<a href="https://github.com/w3c/csswg-drafts/commits/master/css-transitions-1/Overview.bs">change log</a>.

For changes in earlier working drafts:

1.   See the
     <a href="https://www.w3.org/TR/2018/WD-css-transitions-1-20181011/#changes">changes
     section in the 11 October 2018 Working Draft</a>.
1.   See the
     <a href="https://www.w3.org/TR/2017/WD-css-transitions-1-20171130/#changes">changes
     section in the 30 November 2017 Working Draft</a>.
1.   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>.
1.   See the <a href="https://www.w3.org/TR/2013/WD-css3-transitions-20130212/ChangeLog">the
     ChangeLog</a> for changes in previous working drafts.
1.   For more details on these changes, see the version control change logs,
     which are split in many parts because of file renaming:
     *   <a href="https://github.com/w3c/csswg-drafts/commits/master/css-transitions-1/Overview.bs">change log since 2017 October 12</a>,
     *   <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.bs">change log from 2015 January 26 to 2017 October 12</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>.


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>