Overview.bs

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<pre class="metadata">
Title: CSS Transitions Level 1
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 all levels https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&amp;component=Transitions&amp;resolution=---
Test Suite: https://wpt.fyi/results/css/css-transitions
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:css22; 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-4;
 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>

Value Definitions {#values}
-----------------

 This specification follows the <a href="https://www.w3.org/TR/CSS2/about.html#property-defs">CSS property definition conventions</a> from [[!CSS2]]
 using the <a href="https://www.w3.org/TR/css-values-3/#value-defs">value definition syntax</a> from [[!CSS-VALUES-3]].
 Value types not defined in this specification are defined in CSS Values &amp; Units [[!CSS-VALUES-3]].
 Combination with other CSS modules may expand the definitions of these value types.

 In addition to the property-specific values listed in their definitions,
 all properties defined in this specification
 also accept the <a>CSS-wide keywords</a> as their property value.
 For readability they have not been repeated explicitly.

<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 gradual 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="images/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 [0s,∞]>>#
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: <<easing-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 after 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
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>> || <<easing-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="examples/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.