Mercurial > drafts / file revision

 <h1>CSS Transitions</h1>

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 <pre class="metadata">

 Status: ED

 Work Status: Refining

 Shortname: css-transitions

 Group: csswg

 Level: 1

 TR: https://www.w3.org/TR/css3-transitions/

 Previous version: https://www.w3.org/TR/2013/WD-css3-transitions-20131119/

 ED: https://drafts.csswg.org/css-transitions/

 Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/

 Editor: Dean Jackson, Apple Inc https://www.apple.com/, dino@apple.com

 Editor: David Hyatt, Apple Inc https://www.apple.com/, hyatt@apple.com

 Editor: Chris Marrin, Apple Inc https://www.apple.com/, cmarrin@apple.com

 Issue Tracking: Bugzilla bugs for this level https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&product=CSS&component=Transitions&resolution=---&status_whiteboard=defer%20to%20level%202&status_whiteboard_type=notregexp

 Issue Tracking: Bugzilla bugs for all levels https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&product=CSS&component=Transitions&resolution=---

 Abstract: CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.

 Status Text: <strong>This document</strong> is expected to be relatively close to last call.  While some issues raised have yet to be addressed, new features are extremely unlikely to be considered for this level. <p>The following behaviors are at risk: <ul><li><a href="#discrete-interpolation-at-risk">Interpolation in steps of property types that cannot be interpolated</a></li></ul>

 Ignored Terms: float

 Ignored Vars: x1, x2, y1, y2

 Link Defaults: css-transforms (property) transform

 </pre>

 <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>

 </dl>

 Introduction {#introduction}

 ============================

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

       <p>

         This document introduces new CSS features to enable <em>implicit transitions</em>, which describe how CSS properties can be made to change smoothly from one value to another over a given duration.

       </p>

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

 =========================================================

       <p>

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

       </p>

       <p>

         For example, suppose that transitions of one second have been defined on the 'left' and

         'background-color' properties. The following diagram illustrates the effect of updating those properties on an element, in this case moving it to the right and changing the background from red to blue. This assumes other transition parameters still have their default values.

       </p>

       <div class="figure">

         <img src="transition1.png" alt="">

       </div>

       <p class="caption">

         Transitions of 'left' and 'background-color'

       </p>

       <p>

         Transitions are a presentational effect. The <a>computed value</a> of a property transitions over time from the old value to the new value. Therefore if a script queries the <a>computed value</a> of a property (or other data depending on it) as it is transitioning, it will see an intermediate value that represents the current animated value of the property.

       </p>

       <p>

         Only animatable CSS properties can be transitioned. See the table at the end of this document for a list

         of properties that are animatable.

       </p>

       <p>

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

       </p>

       <div class="example">

         <p style="display:none">

           Example(s):

         </p>

         <pre>

   div {

     transition-property: opacity;

     transition-duration: 2s;

   }

   </pre>The above example defines a transition on the 'opacity' property that, when a new value is assigned to it, will cause a smooth change between the old value and the new value over a period of two seconds.

       </div>

       <p>

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

       </p>

       <div class="example">

         <p style="display:none">

           Example(s):

         </p>

         <pre>

   div {

     transition-property: opacity, left;

     transition-duration: 2s, 4s;

   }

   </pre>This will cause the 'opacity' property to transition over a period of two seconds and the left property to transition over a period of four seconds.

       </div>

       <p id="list-matching">

         In the case where the lists of values in transition properties

         do not have the same length, the length of the

         'transition-property' list determines the number of items in

         each list examined when starting transitions.  The lists are

         matched up from the first value: excess values at the end are

         not used.  If one of the other properties doesn't have enough

         comma-separated values to match the number of values of

         'transition-property', the UA must calculate its used value by

         repeating the list of values until there are enough.  This

         truncation or repetition does not affect the computed value.

         <span class="note">

           Note: This is analogous to the behavior of the 'background-*'

           properties, with 'background-image' analogous to

           'transition-property'.

         </span>

       </p>

       <div class="example">

         <p style="display:none">

           Example(s):

         </p>

       <pre>

       div {

         transition-property: opacity, left, top, width;

         transition-duration: 2s, 1s;

       }

       </pre>The above example defines a transition on the 'opacity' property of 2 seconds duration, a

       transition on the 'left' property of 1

       second duration, a transition on the 'top' property of 2 seconds duration and a

       transition on the 'width' property of 1

       second duration.

       </div>

       <p>

         While authors can use transitions to create dynamically changing content,

         dynamically changing content can lead to seizures in some users.

         For information on how to avoid content that can lead to seizures, see

         <a href="https://www.w3.org/TR/WCAG20/#seizure">Guideline 2.3:

         Seizures:

         Do not design content in a way that is known to cause seizures</a>

         ([[WCAG20]]).

       </p>

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

 ----------------------------------------------------------------------------------------------------------------------

       <p>

         The 'transition-property' property specifies the name of the CSS property to which the transition is applied.

       </p>

       <pre class="propdef">

         Name: transition-property

         Value: ''transition-property/none'' | <<single-transition-property>> #

         Initial: ''transition-property/all''

         Applies to: all elements, ::before and ::after pseudo elements

         Inherited: no

         Animatable: no

         Percentages: N/A

         Media: visual

         Computed value: Same as specified value.

         Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>

       </pre>

       <div class="prod">

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

       </div>

       <p>

         A value of

         <dfn value for="transition-property">none</dfn>

         means that no property will transition.

         Otherwise, a list of properties to be transitioned, or the

         keyword <dfn value for="transition-property">all</dfn>

         which indicates that all properties are to be

         transitioned, is given.

       </p>

       <p>

         If one of the identifiers listed is not a recognized property

         name or is not an animatable property, the implementation must

         still start transitions on the animatable properties in the

         list using the duration, delay, and timing function at their

         respective indices in the lists for 'transition-duration',

         'transition-delay', and 'transition-timing-function'.  In other

         words, unrecognized or non-animatable properties must be kept in

         the list to preserve the matching of indices.

       </p>

       <p>

         The <<custom-ident>> production in <<single-transition-property>>

         also excludes the keyword ''transition-property/none'',

         in addition to the keywords always excluded from <<custom-ident>>.

         This means that

         ''transition-property/none'', ''inherit'', and ''initial'' are not

         permitted as items within a list of more that one identifier;

         any list that uses them is syntactically invalid.

       </p>

       <p>

         For the keyword ''transition-property/all'',

         or if one of the identifiers listed is a

         shorthand property, implementations must start transitions for

         any of its longhand sub-properties that are animatable (or, for

         ''transition-property/all'', all animatable properties), using the duration, delay,

         and timing function at the index corresponding to the shorthand.

       </p>

       <p>

         If a property is specified multiple times in the value of

         'transition-property' (either on its own, via a shorthand that

         contains it, or via the ''transition-property/all'' value), then the transition that

         starts uses the duration, delay, and timing function at the

         index corresponding to the <em>last</em> item in the value of

         'transition-property' that calls for animating that property.

       </p>

       <p class="note">

         Note:  The ''transition-property/all'' value and 'all' shorthand

         property work in similar ways, so the

         ''transition-property/all'' value is just like a shorthand that

         covers all properties.

       </p>

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

 ----------------------------------------------------------------------------------------------------------------------

       <p>

         The 'transition-duration' property defines the length of time that a transition takes.

       </p>

       <pre class="propdef">

         Name: transition-duration

         Value: <<time>> #

         Initial: ''0s''

         Applies to: all elements, ::before and ::after pseudo elements

         Inherited: no

         Animatable: no

         Percentages: N/A

         Media: interactive

         Computed value: Same as specified value.

         Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>

       </pre>

       <p>

         This property specifies how long the transition from the old value to the new value should take. By default the value is ''0s'', meaning that the transition is immediate (i.e. there will be no animation). A negative value for 'transition-duration' renders the declaration invalid.

       </p>

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

 ---------------------------------------------------------------------------------------------------------------------------------

       <p>

         The 'transition-timing-function' property

         describes how the intermediate values used during a transition will be

         calculated. It allows for a transition to change speed over its

         duration. These effects are commonly called <em>easing</em> functions.

         In either case, a mathematical function that provides a smooth curve is

         used.

       </p>

       <p>

         Timing functions are either defined as a stepping function or

         a <a

         href="http://en.wikipedia.org/wiki/B%C3%A9zier_curve#Cubic_B.C3.A9zier_curves">cubic

         B&eacute;zier curve</a>.

         The timing function takes as its input

         the current elapsed percentage of the transition duration

         and outputs the percentage of the way the transition is

         from its start value to its end value.

         How this output is used is defined by

         the <a href="#animatable-types">interpolation rules</a>

         for the value type.

       </p>

       <p>

         A <a href="http://en.wikipedia.org/wiki/Step_function">stepping</a>

         function is defined by a number that divides the domain of operation

         into equally sized intervals. Each subsequent interval is a equal step

         closer to the goal state. The function also specifies whether the

         change in output percentage happens at the start or end of the

         interval (in other words, if 0% on the input percentage is the point

         of initial change).

       </p>

       <div class="figure">

         <img src="step.png" alt="The step timing function splits

           the function domain into a number of disjoint straight line

           segments. steps(1, start) is a function whose

           output value is 1 for all input values. steps(1, end) is a function whose

           output value is 0 for all input values less than 1, and output

           is 1 for the input value of 1. steps(3, start) is a function that

           divides the input domain into three segments, each 1/3 in length,

           and 1/3 above the previous segment, with the first segment starting

           at 1/3. steps(3, end) is a function that

           divides the input domain into three segments, each 1/3 in length,

           and 1/3 above the previous segment, with the first segment starting

           at 0.">

       </div>

       <p class="caption">

         Step timing functions

       </p>

       <p>

         A <a

         href="http://en.wikipedia.org/wiki/B%C3%A9zier_curve#Cubic_B.C3.A9zier_curves">cubic

         B&eacute;zier curve</a> is defined by four control points, P<sub>0</sub>

         through P<sub>3</sub> (see Figure 1). P<sub>0</sub> and P<sub>3</sub>

         are always set to (0,0) and (1,1). The 'transition-timing-function' property is used

         to specify the values for points P<sub>1</sub> and P<sub>2</sub>. These

         can be set to preset values using the keywords listed below, or can be

         set to specific values using the ''cubic-bezier()'' function.

         In the ''cubic-bezier()'' function, P<sub>1</sub> and

         P<sub>2</sub> are each specified by both an X and Y value.

       </p>

       <div class="figure">

         <img src="TimingFunction.png" alt="The B&eacute;zier timing function is a

           smooth curve from point P0 = (0,0) to point P3 = (1,1). The

           length and orientation of the line segment P0-P1 determines

           the tangent and the curvature of the curve at P0 and the

           line segment P2-P3 does the same at P3.">

       </div>

       <p class="caption">

         Bézier Timing Function Control Points

       </p>

       <pre class="propdef">

         Name: transition-timing-function

         Value: <<single-transition-timing-function>> #

         Initial: ''transition-timing-function/ease''

         Applies to: all elements, ::before and ::after pseudo elements

         Inherited: no

         Animatable: no

         Percentages: N/A

         Media: interactive

         Computed value: Same as specified value.

         Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>

       </pre>

       <div class="prod">

         <dfn type id="single-transition-timing-function">&lt;single-transition-timing-function&gt;</dfn> = ''ease'' | ''linear'' | ''ease-in'' | ''ease-out'' | ''ease-in-out'' | ''step-start'' | ''step-end'' | <a lt="steps()" function>steps</a>(<<integer>>[, [ ''start'' | ''end'' ] ]?) | <a lt="cubic-bezier()" function>cubic-bezier</a>(<<number>>, <<number>>, <<number>>, <<number>>)

       </div>

       <p>

         The timing functions have the following definitions.

       </p>

       <dl dfn-type="value" dfn-for="transition-timing-function, <single-transition-timing-function>">

         <dt><dfn>ease</dfn></dt>

         <dd>

           The ease function is equivalent to <a lt="cubic-bezier()" function>cubic-bezier(0.25, 0.1, 0.25, 1)</a>.

         </dd>

         <dt><dfn>linear</dfn></dt>

         <dd>

           The linear function is equivalent to <a lt="cubic-bezier()" function>cubic-bezier(0, 0, 1, 1)</a>.

         </dd>

         <dt><dfn>ease-in</dfn></dt>

         <dd>

           The ease-in function is equivalent to <a lt="cubic-bezier()" function>cubic-bezier(0.42, 0, 1, 1)</a>.

         </dd>

         <dt><dfn>ease-out</dfn></dt>

         <dd>

           The ease-out function is equivalent to <a lt="cubic-bezier()" function>cubic-bezier(0, 0, 0.58, 1)</a>.

         </dd>

         <dt><dfn>ease-in-out</dfn></dt>

         <dd>

           The ease-in-out function is equivalent to <a lt="cubic-bezier()" function>cubic-bezier(0.42, 0, 0.58, 1)</a>.

         </dd>

         <dt><dfn>step-start</dfn></dt>

         <dd>

           The step-start function is equivalent to <a lt="steps()" function>steps(1, start)</a>.

         </dd>

         <dt><dfn>step-end</dfn></dt>

         <dd>

           The step-end function is equivalent to <a lt="steps()" function>steps(1, end)</a>.

         </dd>

         <dt><dfn function lt="steps()">steps(<<integer>>[, [ start | end ] ]?)</dfn></dt>

         <dd>

           Specifies a stepping function, described above, taking two

           parameters. The first parameter specifies the number of intervals

           in the function. It must be a positive integer (greater than 0).

           The second parameter, which is optional, is

           either the value <dfn value for="steps()">start</dfn> or <dfn value for="steps()">end</dfn>, and specifies the point

           at which the change of values occur within the interval.

           If the second parameter is omitted, it is given the value ''end''.

         </dd>

         <dt><dfn function lt="cubic-bezier()">cubic-bezier(<<number>>, <<number>>, <<number>>, <<number>>)</dfn></dt>

         <dd>

           Specifies a <a

           href="http://en.wikipedia.org/wiki/B%C3%A9zier_curve">cubic-bezier

           curve</a>. The four values specify points P<sub>1</sub> and

           P<sub>2</sub> of the curve as (<var>x1</var>, <var>y1</var>, <var>x2</var>, <var>y2</var>). Both x values must be

           in the range [0, 1] or the definition is invalid. The y values can

           exceed this range.

         </dd>

       </dl>

 ### Serializing a timing function ### {#serializing-a-timing-function}

       Timing functions are serialized using the common serialization patterns

       defined in [[CSSOM]] with the following additional requirements:

       *   The keyword values ''ease'', ''linear'', ''ease-in'', ''ease-out'',

           and ''ease-in-out'' are serialized as-is, that is, they are

           <em>not</em> converted to the equivalent ''cubic-bezier()''

           function before serializing.

       *   Step timing functions, whether they are specified using the

           ''steps()'' function or either of the ''step-start'' or ''step-end''

           keywords, are serialized as follows:

           1.   If the point at which the value changes is ''end'', serialize

                as <a lt="steps()" function>steps(&lt;integer&gt;)</a>.

           2.   Otherwise, serialize as <a lt="steps()"

                function>steps(&lt;integer&gt;, start)</a>.

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

 -------------------------------------------------------------------------------------------------------------

       <p>

         The 'transition-delay' property defines when the transition will start. It allows a transition to begin execution some some period of time from when it is applied. A 'transition-delay' value of ''0s'' means the transition will execute as soon as the property is changed. Otherwise, the value specifies an offset from the moment the property is changed, and the transition will delay execution by that offset.

       </p>

       <p>

         If the value for 'transition-delay' is a negative time offset then the transition will execute the moment the property is changed, but will appear to have begun execution at the specified offset. That is, the transition will appear to begin part-way through its play cycle. In the case where a transition has implied starting values and a negative 'transition-delay', the starting values are taken from the moment the property is changed.

       </p>

       <pre class="propdef">

         Name: transition-delay

         Value: <<time>> #

         Initial: ''0s''

         Applies to: all elements, ::before and ::after pseudo elements

         Inherited: no

         Animatable: no

         Percentages: N/A

         Media: interactive

         Computed value: Same as specified value.

         Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>

       </pre>

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

 -------------------------------------------------------------------------------------------------------------------------

       <p>

         The 'transition' shorthand property combines the four properties described above into a single property.

       </p>

       <pre class="propdef">

         Name: transition

         Value: <<single-transition>> #

         Initial: see individual properties

         Applies to: all elements, ::before and ::after pseudo elements

         Inherited: no

         Animatable: no

         Percentages: N/A

         Media: interactive

         Computed value: see individual properties

         Canonical order: <abbr title="follows order of property value definition">per grammar</abbr>

       </pre>

       <div class="prod">

         <dfn type id="single-transition">&lt;single-transition&gt;</dfn> = [ ''transition-property/none'' | <<single-transition-property>> ] || <<time>> || <<single-transition-timing-function>> || <<time>>

       </div>

       <p>

         Note that order is important within the items in this property:

         the first value that can be parsed as a time is assigned to the

         transition-duration,

         and the second value that can be parsed as a time is assigned to

         transition-delay.

       </p>

       <p>

         If there is more than one <<single-transition>> in the shorthand,

         and any of the transitions has

         ''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 even or was not in the document during

         the previous style change event,

         then transitions are not started for that element

         in that style change event.

         Otherwise,

         define the <dfn export>before-change style</dfn> as

         the <a>computed values</a> of all properties on the element as of

         the previous <a>style change event</a>,

         except with any styles derived from declarative

         animations such as CSS Transitions, CSS Animations

         ([[CSS3-ANIMATIONS]]),

         and SMIL Animations ([[SMIL-ANIMATION]], [[SVG11]])

         updated to the current time.

         Likewise, define the <dfn export>after-change style</dfn> as

         the <a>computed values</a> of all properties

         on the element based on the information

         known at the start of that <a>style change event</a>,

         but excluding any styles from CSS Transitions in the computation,

         and inheriting from

         the <a>after-change style</a> of the parent.

       </p>

       <div class="note">

         <p>

           Note that this definition of the <a>after-change style</a>

           means that a single change

           can start a transition on the same property

           on both an ancestor element and its descendant element.

           This can happen when a property change is inherited

           from one element with 'transition-*' properties

           that say to animate the changing property

           to another element with 'transition-*' properties

           that also say to animate the changing property.

         </p>

         <p>

           When this happens, both transitions will run,

           and the transition on the descendant will override

           the transition on the ancestor

           because of the normal

           CSS cascading and inheritance rules ([[CSS3CASCADE]]).

         </p>

         <p>

           If the transition on the descendant completes before

           the transition on the ancestor,

           the descendant will then resume inheriting

           the (still transitioning) value from its parent.

           This effect is likely not a desirable effect,

           but it is essentially doing what the author asked for.

         </p>

       </div>

       <p>

         For each element with a <a>before-change style</a> and

         an <a>after-change style</a>,

         and each property (other than shorthands),

         define the <dfn export>matching transition-property value</dfn> as

         the last value in the

         'transition-property' in the element's <a>after-change style</a>

         that matches the property,

         as described in

         [[#transition-property-property]].

         If there is such a value, then corresponding to it, there is

         a <dfn export>matching transition duration</dfn>,

         a <dfn export>matching transition delay</dfn>, and

         a <dfn export>matching transition timing function</dfn>

         in the values in the <a>after-change style</a> of

         'transition-duration', 'transition-delay', and 'transition-timing-function'

         (see <a href="#list-matching">the rules on matching lists</a>).

         Define the <dfn export for="transition">combined duration</dfn> of the transition

         as the sum of max(<a>matching transition duration</a>, ''0s'') and

         the <a>matching transition delay</a>.

         For each element and property, the implementation must act

         as follows:

       </p>

       <ol>

       <li>

         If all of the following are true:

         <ul>

           <li>

             the element does not have

             a <a>running transition</a> for the property,

           </li>

           <li>

             the <a>before-change style</a> is different from

             and can be interpolated with

             the <a>after-change style</a> for that property,

           </li>

           <li>

             the element does not have a <a>completed transition</a>

             for the property

             or the <a>end value</a> of the <a>completed transition</a>

             is different from the <a>after-change style</a> for the property,

           </li>

           <li>

             there is a <a>matching transition-property value</a>, and

           </li>

           <li>

             the <a>combined duration</a> is greater than ''0s'',

           </li>

         </ul>

         then implementations must

         remove the <a>completed transition</a> (if present) from the set

         of completed transitions and

         start a transition whose:

         <ul>

           <li>

             <a>start time</a> is

             the time of the <a>style change event</a> plus

             the <a>matching transition delay</a>,

           </li>

           <li>

             <a>end time</a> is

             the <a>start time</a> plus

             the <a>matching transition duration</a>,

           </li>

           <li>

             <a>start value</a> is

             the value of the transitioning property

             in the <a>before-change style</a>,

           </li>

           <li>

             <a>end value</a> is

             the value of the transitioning property

             in the <a>after-change style</a>,

           </li>

           <li>

             <a>reversing-adjusted start value</a> is the same as

             the <a>start value</a>, and

           <li>

             <a>reversing shortening factor</a> is 1.

           </li>

         </ul>

       </li>

       <li>

         Otherwise,

         if the element has a <a>completed transition</a> for the property

         and the <a>end value</a> of the <a>completed transition</a>

         is different from the <a>after-change style</a> for the property,

         then implementations must

         remove the <a>completed transition</a> from the set of

         <a>completed transitions</a>.

       </li>

       <li>

         If the element has a <a>running transition</a> or

         <a>completed transition</a> for the property,

         and there is <strong>not</strong>

         a <a>matching transition-property value</a>,

         then implementations must

         <a>cancel</a> the <a>running transition</a>

         or remove the <a>completed transition</a> from the set of

         <a>completed transitions</a>.

       </li>

       <li>

         If the element has a <a>running transition</a> for the property,

         there is a <a>matching transition-property value</a>,

         and the <a>end value</a> of the <a>running transition</a> is

         <strong>not</strong> equal to the value of the property in the

         <a>after-change style</a>, then:

         <ol>

           <li>

             If the <a>current value</a> of the property

             in the <a>running transition</a>

             is equal to

             the value of the property in the <a>after-change style</a>,

             or if these two values cannot be interpolated,

             then implementations must

             <a>cancel</a> the <a>running transition</a>.

           </li>

           <li>

             Otherwise, if the <a>combined duration</a> is

             less than or equal to ''0s'',

             or if the

             <a>current value</a> of the property in the <a>running transition</a>

             cannot be interpolated with

             the value of the property in the <a>after-change style</a>,

             then implementations must

             <a>cancel</a> the <a>running transition</a>.

           </li>

           <li>

             Otherwise, if the <a>reversing-adjusted start value</a>

             of the <a>running transition</a> is the same as the value of

             the property in the <a>after-change style</a>

             <span class="note">(see the

             <a href="#reversing">section on reversing of

             transitions</a> for why these case exists)</span>,

             implementations must

             <a>cancel</a> the <a>running transition</a> and

             start a new transition whose:

             <ul>

               <li>

                 <a>reversing-adjusted start value</a> is

                 the <a>end value</a> of the

                 <a>running transition</a>

                 <span class="note">(Note: This represents the logical start state of

                 the transition, and allows some calculations to ignore that

                 the transition started before that state was reached, which

                 in turn allows repeated reversals of the same transition to

                 work correctly),</span>

               <li>

                 <a>reversing shortening factor</a>

                 is the absolute value, clamped to the range [0, 1],

                 of the sum of:

                 <ol>

                   <li>the output of the timing function of the old transition

                   at the time of the <a>style change event</a>,

                   times the <a>reversing shortening factor</a> of the

                   old transition</li>

                   <li>1 minus the <a>reversing shortening factor</a> of

                   the old transition.</li>

                 </ol>

                 <span class="note">Note: This represents the portion of the

                 space between the <a>reversing-adjusted start value</a>

                 and the <a>end value</a> that the old transition has

                 traversed (in amounts of the value, not time), except with the

                 absolute value and clamping to handle timing functions that

                 have y1 or y2 outside the range [0, 1].</span>

               </li>

               <li>

                 <a>start time</a> is

                 the time of the <a>style change event</a> plus:

                 <ol>

                   <li>if the <a>matching transition delay</a>

                       is nonnegative,

                       the <a>matching transition delay</a>, or

                   <li>if the <a>matching transition delay</a>

                       is negative,

                       the product of

                       the new transition's

                       <a>reversing shortening factor</a> and

                       the <a>matching transition delay</a>,

                 </ol>

               </li>

               <li>

                 <a>end time</a> is

                 the <a>start time</a> plus the product of

                 the <a>matching transition duration</a> and

                 the new transition's <a>reversing shortening factor</a>,

               </li>

               <li>

                 <a>start value</a> is

                 the <a>current value</a> of the property

                 in the <a>running transition</a>,

               </li>

               <li>

                 <a>end value</a> is

                 the value of the property

                 in the <a>after-change style</a>,

               </li>

             </ul>

           </li>

           <li>

             Otherwise, implementations must

             <a>cancel</a> the <a>running transition</a>

             and start a new transition whose:

             <ul>

               <li>

                 <a>start time</a> is

                 the time of the <a>style change event</a> plus

                 the <a>matching transition delay</a>,

               </li>

               <li>

                 <a>end time</a> is

                 the <a>start time</a> plus

                 the <a>matching transition duration</a>,

               </li>

               <li>

                 <a>start value</a> is

                 the <a>current value</a> of the property

                 in the <a>running transition</a>,

               </li>

               <li>

                 <a>end value</a> is

                 the value of the property

                 in the <a>after-change style</a>,

               </li>

               <li>

                 <a>reversing-adjusted start value</a> is the same as

                 the <a>start value</a>, and

               <li>

                 <a>reversing shortening factor</a> is 1.

               </li>

             </ul>

           </li>

         </ol>

       </li>

       </ol>

       <div class="note">

         <p>

           Note that the above rules mean that

           when the computed value of an animatable property changes,

           the transitions that start are based on the

           values of the 'transition-property', 'transition-duration',

           'transition-timing-function', and 'transition-delay' properties

           at the time the animatable property would first have its new

           computed value.

           This means that when one of these 'transition-*' properties

           changes at the same time as

           a property whose change might transition,

           it is the <em>new</em> values of the 'transition-*' properties

           that control the transition.

         </p>

         <div class="example" id="manual-reversing-example">

           <p style="display:none">

             Example(s):

           </p>

           <p>This provides a way for authors to specify different values

           of the 'transition-*' properties for the “forward”

           and “reverse” transitions,

           when the transitions are between two states

           (but see <a

           href="#reversing">below</a> for special reversing behavior when

           an <em>incomplete</em> transition is interrupted).  Authors can

           specify the value of 'transition-duration',

           'transition-timing-function', or 'transition-delay' in the same

           rule where they specify the value that triggers the transition,

           or can change these properties at the same time as they change

           the property that triggers the transition.  Since it's the new

           values of these 'transition-*' properties that affect the

           transition, these values will be used for the transitions

           <em>to</em> the associated transitioning values.  For example:

            </p>

           <pre>

   li {

     transition: background-color linear 1s;

     background: blue;

   }

   li:hover {

     background-color: green;

     transition-duration: 2s; /* applies to the transition *to* the :hover state */

   }</pre>

           <p>

             When a list item with these style rules enters the :hover

             state, the computed 'transition-duration' at the time that

             'background-color' would have its new value (''green'') is ''2s'',

             so the transition from ''blue'' to ''green'' takes 2 seconds.

             However, when the list item leaves the :hover state, the

             transition from ''green'' to ''blue'' takes 1 second.

           </p>

         </div>

       </div>

       <p class="note">

         Note that once the transition of a property has started

         (including being in its delay phase),

         it continues running based on

         the original timing function, duration, and

         delay, even if the 'transition-timing-function',

         'transition-duration', or 'transition-delay' property changes

         before the transition is complete.  However, if the

         'transition-property' property changes such that the transition

         would not have started, the transition stops (and the

         property immediately changes to its final value).

       </p>

       <p class="note">

         Note that above rules mean that

         transitions do not start when the computed

         value of a property changes as a result of declarative animation

         (as opposed to scripted animation).

         This happens because the before-change style includes up-to-date

         style for declarative animations.

       </p>

 Faster reversing of interrupted transitions {#reversing}

 --------------------------------------------------------

       <div class="note">

       <p>

         Many common transitions effects involve transitions between two states,

         such as the transition that occurs when the mouse pointer moves

         over a user interface element, and then later moves out of that element.

         With these effects, it is common for a running transition

         to be interrupted before it completes,

         and the property reset to the starting value of that transition.

         An example is a hover effect on an element,

         where a transition starts when the pointer enters the element,

         and then the pointer exits the element before the effect has completed.

         If the outgoing and incoming transitions

         are executed using their specified durations and timing functions,

         the resulting effect can be distractingly asymmetric

         because the second transition

         takes the full specified time to move a shortened distance.

         Instead, this specification makes second transition shorter.

       </p>

       <p>

         The mechanism the above rules use to cause this involves the

         <a>reversing shortening factor</a> and the

         <a>reversing-adjusted start value</a>.

         In particular, the reversing behavior is present whenever

         the <a>reversing shortening factor</a> is less than 1.

       </p>

       <p class="note">

         Note that these rules do not fully address the problem for

         transition patterns that involve more than two states.

       </p>

       <p class="note">

         Note that these rules lead to the entire timing function of the

         new transition being used, rather than jumping into the middle

         of a timing function, which can create a jarring effect.

       </p>

       <p class="note">

         This was one of several possibilities that was considered by the

         working group.  See the

         <a href="transition-reversing-demo">reversing demo</a>

         demonstrating a number of them, leading to a working group

         resolution made on 2013-06-07 and edits made on 2013-11-11.

       </p>

       </div>

 Application of transitions {#application}

 =========================================

       <p>

         When a property on an element is undergoing a transition

         (that is, when or after the transition has started and before the

         <a>end time</a> of the transition)

         the transition adds a style called the <dfn export>current value</dfn>

         to the CSS cascade

         at the level defined for CSS Transitions in [[!CSS3CASCADE]].

       </p>

       <p class="note">

         Note that this means that computed values

         resulting from CSS transitions

         can inherit to descendants just like

         any other computed values.

         In the normal case, this means that

         a transition of an inherited property

         applies to descendant elements

         just as an author would expect.

       </p>

       <p>

         Implementations must add this value to the cascade

         if and only if

         that property is not currently

         undergoing a CSS Animation ([[!CSS3-ANIMATIONS]]) on the same element.

       </p>

       <p class="note">

         Note that this behavior of transitions not applying to the cascade

         when an animation on the same element and property is running

         does not affect whether the transition has started or ended.

         APIs that detect whether transitions are running

         (such as <a href="#transition-events">transition events</a>)

         still report that a transition is running.

       </p>

       <p>

         If the current time is at or before the

         <a>start time</a> of the transition

         (that is, during the delay phase of the transition),

         the <a>current value</a> is a specified style that will compute

         to the <a>start value</a> of the transition.

       </p>

       <p>

         If the current time is after the

         <a>start time</a> of the transition

         (that is, during the duration phase of the transition),

         the <a>current value</a> is a specified style that will compute

         to the <a href="#animatable-types">result of interpolating the property</a>

         using the <a>start value</a> of the transition as

         <var>V</var><sub>start</sub>,

         using the <a>end value</a> of the transition as

         <var>V</var><sub>end</sub>,

         and using (current time - start time) / (end time - start time)

         as the input to the timing function.

       </p>

 Completion of transitions {#complete}

 =====================================

       <p>

         <a>Running transitions</a>

         <dfn export for="transition" id="dfn-complete">complete</dfn>

         at a time that equal to or after their end time,

         but prior to to the first <a>style change event</a>

         whose time is equal to or after their <a>end time</a>.

         When a transition completes,

         implementations must move

         all transitions that complete at that time

         from the set of <a>running transitions</a>

         to the set of <a>completed transitions</a>

         and then fire the <a href="#transition-events">events</a>

         for those completions.

         <span class="note">(Note that doing otherwise, that is,

         firing some of the events before doing all of the moving

         from <a>running transitions</a> to <a>completed transitions</a>,

         could allow

         a style change event to happen

         without the necessary transitions completing,

         since firing the event could cause a style change event,

         if an event handler requests up-to-date computed style or

         layout data.)</span>

       </p>

 <span id="transition-events-">Transition Events</span> {#transition-events}

 ===========================================================================

 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">

   [Constructor(DOMString type, optional TransitionEventInit transitionEventInitDict)]

   interface TransitionEvent : Event {

     readonly attribute DOMString propertyName;

     readonly attribute float elapsedTime;

     readonly attribute DOMString pseudoElement;

   };

   dictionary TransitionEventInit : EventInit {

     DOMString propertyName = "";

     float elapsedTime = 0.0;

     DOMString pseudoElement = "";

   };

 </pre>

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

 :   <code class='attribute-name'><dfn attribute for="TransitionEvent" id="Events-TransitionEvent-propertyName">propertyName</dfn></code> of type <code>DOMString</code>, readonly

 ::  The name of the CSS property associated with the transition.

 :   <code class='attribute-name'><dfn attribute for="TransitionEvent" id="Events-TransitionEvent-elapsedTime">elapsedTime</dfn></code> of type <code>float</code>, readonly

 ::  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 precise

     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> of type <code>DOMString</code>, readonly

 ::  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">cancelled</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 cancelled.

       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><dfn>ontransitionrun</dfn></td>

     <td><a idl>transitionrun</a></td>

   </tr>

   <tr>

     <td><dfn>ontransitionstart</dfn></td>

     <td><a idl>transitionstart</a></td>

   </tr>

   <tr>

     <td><dfn>ontransitionend</dfn></td>

     <td><a idl>transitionend</a></td>

   </tr>

   <tr>

     <td><dfn>ontransitioncancel</dfn></td>

     <td><a idl>transitioncancel</a></td>

   </tr>

 </table>

 DOM Interfaces {#interface-dom}

 ===============================

 This specification extends the {{GlobalEventHandlers}} interface 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 GlobalEventHandlers {

   attribute EventHandler ontransitionrun;

   attribute EventHandler ontransitionstart;

   attribute EventHandler ontransitionend;

   attribute EventHandler ontransitioncancel;

 };

 </pre>

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

 ==============================================================================================

       <p>

         Some property types can be interpolated,

         which means they can animate smoothly from one value to another.

         Other property types cannot, and thus animate only in a single

         step from one value to the other.

       </p>

 Animation of interpolated property types {#interpolated-types}

 --------------------------------------------------------------

       <p>

         When interpolating between two values,

         <var>V</var><sub>start</sub> and <var>V</var><sub>end</sub>,

         interpolation is done using the output <var>p</var> of the timing function,

         which gives the portion of the value space

         that the interpolation has crossed.

         Thus the result of the interpolation is

         <var>V</var><sub>res</sub> =

           (1 - <var>p</var>) &sdot; <var>V</var><sub>start</sub> +

           <var>p</var> &sdot; <var>V</var><sub>end</sub>.

       </p>

       <p>

         However, if this value (<var>V</var><sub>res</sub>)

         is outside the allowed range of values for the property,

         then it is clamped to that range.

         This can occur if <var>p</var> is outside of the range 0 to 1,

         which can occur if a timing function is specified

         with a <var>y1</var> or <var>y2</var> that is outside the range 0 to 1.

       </p>

       <p>

         The following describes how each property type undergoes transition or

         animation.

       </p>

       <ul>

         <li id="animtype-color">

           <strong>color</strong>: interpolated via red, green, blue and alpha

           components (treating each as a number, see below).

           The interpolation is done between premultiplied colors

           (that is, colors for which the red, green, and blue components

           specified have been multiplied by the alpha).

         </li>

         <li id="animtype-length">

           <strong>length</strong>: interpolated as real numbers.

         </li>

         <li id="animtype-percentage">

           <strong>percentage</strong>: interpolated as real numbers.

         </li>

         <li id="animtype-lpcalc">

           <strong>length, percentage, or calc</strong>: when both values

           are lengths, interpolated as lengths; when both values are

           percentages, interpolated as percentages; otherwise, both

           values are converted into a ''calc()'' function that is the

           sum of a length and a percentage (each possibly zero), and

           these ''calc()'' functions have each half interpolated as real

           numbers.

         </li>

         <li id="animtype-integer">

           <strong>integer</strong>: interpolated via discrete steps (whole

           numbers). The interpolation happens in real number space and is

           converted to an integer by rounding to the nearest integer, with

           values halfway between a pair of integers rounded towards

           positive infinity.

         </li>

         <li id="animtype-font-weight">

           <strong>font weight</strong>: interpolated via discrete steps

           (multiples of 100). The interpolation happens in real number

           space and is converted to an integer by rounding to the

           nearest multiple of 100, with values halfway between multiples

           of 100 rounded towards positive infinity.

         </li>

         <li id="animtype-number">

           <strong>number</strong>: interpolated as real (floating point)

           numbers.

         </li>

         <li id="animtype-rect">

           <strong>rectangle</strong>: interpolated via the x, y,

           width and height components (treating each as a number).

         </li>

         <li id="animtype-visibility">

           <strong>visibility</strong>: if one of the values is

           ''visibility/visible'', interpolated as a discrete step where values of the

           timing function between 0 and 1 map to ''visibility/visible'' and other

           values of the timing function (which occur only at the

           start/end of the transition or as a result of ''cubic-bezier()''

           functions with Y values outside of [0, 1]) map to the closer

           endpoint; if neither value is ''visibility/visible'' then not interpolable.

         </li>

         <li id="animtype-shadow-list">

           <strong>shadow list</strong>: Each shadow in the list

           (treating ''shadow/none'' as a 0-length list)

           is interpolated via the

           color (as <a href="#animtype-color">color</a>) component,

           and x, y, blur, and (when appropriate) spread

           (as <a href="#animtype-length">length</a>) components.

           For each shadow, if both input shadows are ''shadow/inset''

           or both input shadows are not ''shadow/inset'',

           then the interpolated shadow must match the input shadows in that regard.

           If any pair of input shadows has one ''shadow/inset'' and the other not ''shadow/inset'',

           the entire <a href="#animtype-shadow-list">shadow-list</a> is uninterpolable.

           If the lists of shadows have different lengths,

           then the shorter list is padded at the end

           with shadows whose color is ''transparent'',

           all lengths are ''0'',

           and whose ''shadow/inset'' (or not) matches the longer list.

         </li>

         <li id="animtype-gradient">

           <strong>gradient</strong>: interpolated via the

           positions and colors of each stop. They must have the same type

           (radial or linear) and same number of stops in order to be animated.

           <span class="note">Note: [[CSS3-IMAGES]] may extend this

           definition.</span>

         </li>

         <li id="animtype-paintserver">

           <strong>paint server</strong> (SVG): interpolation is only supported

           between: gradient to gradient and color to color. They then

           work as above.

         </li>

         <li id="animtype-simple-list">

           <strong>simple list</strong> of other types:

           If the lists have the same number of items,

           and each pair of values can be interpolated,

           each item in the list is interpolated using

           the rules given for those types.

           Otherwise the values are not interpolable.

         </li>

         <li id="animtype-repeatable-list">

           <strong>repeatable list</strong> of other types:

           The result list has a length that is the least common multiple

           of the lengths of the input lists.

           Each item in the result is the interpolation of the value

           from each input list repeated to the length of the result list.

           If a pair of values cannot be interpolated, then the lists

           are not interpolable.

           <span class="note">

             The repeatable list concept ensures that a list that is

             conceptually repeated to a certain length (as

             'background-origin' is repeated to the length of the

             'background-image' list) or repeated infinitely will

             smoothly transition between any values, and so that the

             computed value will properly represent the result (and

             potentially be inherited correctly).

           </span>

         </li>

       </ul>

       <p>Future specifications may define additional types that can

       be animated.</p>

       <p>See the definition of 'transition-property' for how animation

       of shorthand properties and the ''all'' value is applied to any

       properties (in the shorthand) that can be animated.</p>

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

 --------------------------------------------------------

       <p>

         When interpolating between two values that cannot be interpolated,

         <var>V</var><sub>start</sub> and <var>V</var><sub>end</sub>,

         interpolation is done using the output <var>p</var> of the timing function.

         If <var>p</var> is less than 0.5, then the

         result of the interpolation is

         <var>V</var><sub>start</sub>;

         if <var>p</var> is greater than or equal to 0.5, then the

         result of the interpolation is

         <var>V</var><sub>end</sub>.

       </p>

       <p class="note" id="discrete-interpolation-at-risk">

         This is a recent change to which implementations have

         not yet updated.  (Prior to the change CSS Transitions

         and CSS Animations did not run on such changes.)  It's

         possible that it won't be compatible with existing Web content.

         If that is the case, the problem may be mitigated by restricting

         this behavior only to CSS Animations (and not to CSS Transitions),

         and/or restricting it to step timing functions.

       </p>

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

 =======================================================================================

       <!--

       As resolved in

       http://lists.w3.org/Archives/Public/www-style/2011Sep/0497.html

       -->

       <p>The definition of each CSS property defines

       when the values of that property can be interpolated

       by referring to the definitions of property types

       in the <a href="#animatable-types">previous section</a>.

       The animated value is interpolated from the from and to values when

       both the from and the to values of the property have the type described.

       (When a composite type such as "length, percentage, or calc" is listed,

       this means that both values must fit into that composite type.)

       When multiple types are listed in the form "either A or B",

       both values must be of the same type to be interpolable.</p>

       <p>Otherwise, since the from and to values cannot be interpolated,

       the animation is done <a href="#step-types">in a single step</a>.</p>

       <p>The 'transition-*' properties defined in this specification do

       not undergo transitions.</p>

       <p>For properties that exist at the time this specification was

       developed, this specification defines how they are

       animated.  However, future CSS specifications may define

       additional properties, additional values for existing properties,

       or additional animation behavior of existing values.  In order to

       describe new animation behaviors and to have the definition of

       animation behavior in a more appropriate location, future CSS

       specifications should include an "Animatable:" line in the summary

       of the property's definition (in addition to the other lines

       described in [[CSS21]], <a

       href="https://www.w3.org/TR/CSS21/about.html#property-defs">section

       1.4.2</a>).  This line should say "no" to indicate that a property

       cannot be animated or should reference an animation behavior

       (which may be one of the behaviors in the <a

       href="#animation-of-property-types-">Animation of property

       types</a> section above, or may be a new behavior) to define how

       the property animates.  Such definitions override those given in

       this specification.</p>

       <p class="issue" id="issue-animatable-name">

         It no longer makes sense for this line to be called

         "Animatable".  It should probably be renamed to "Interpolation",

         and the "no" value renamed to "discrete" or "in steps".

         See mailing list thread:

         <a href="https://lists.w3.org/Archives/Public/www-style/2015May/0256.html">message 1</a>,

         <a href="https://lists.w3.org/Archives/Public/www-style/2015May/0257.html">message 2</a>

       </p>

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

 ----------------------------------------------------------------------------

       <p>

       The following definitions define the animation behavior for

       properties in CSS Level 2 Revision 1 ([[CSS21]]) and in Level 3 of

       the CSS Color Module ([[CSS3COLOR]]).

       </p>

      <table class="animatable-properties">

        <tr>

          <th>Property Name</th>

          <th>Type</th>

        </tr>

        <tr>

          <td>'background-color'</td><td>as <a href="#animtype-color">color</a></tr>

        <tr>

          <td>'background-position'</td><td>as <a href="#animtype-repeatable-list">repeatable list</a> of <a href="#animtype-simple-list">simple list</a> of <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'border-bottom-color'</td><td>as <a href="#animtype-color">color</a></td>

        </tr>

        <tr>

          <td>'border-bottom-width'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'border-left-color'</td><td>as <a href="#animtype-color">color</a></td>

        </tr>

        <tr>

          <td>'border-left-width'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'border-right-color'</td><td>as <a href="#animtype-color">color</a></td>

        </tr>

        <tr>

          <td>'border-right-width'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'border-spacing'</td><td>as <a href="#animtype-simple-list">simple list</a> of <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'border-top-color'</td><td>as <a href="#animtype-color">color</a></td>

        </tr>

        <tr>

          <td>'border-top-width'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'bottom'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'clip'</td><td>as <a href="#animtype-rect">rectangle</a></td>

        </tr>

        <tr>

          <td>'color'</td><td>as <a href="#animtype-color">color</a></td>

        </tr>

        <tr>

          <td>'font-size'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'font-weight!!property'</td><td>as <a href="#animtype-font-weight">font weight</a></td>

        </tr>

        <tr>

          <td>'height'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'left'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'letter-spacing'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'line-height'</td><td>as either <a href="#animtype-number">number</a> or <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'margin-bottom'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'margin-left'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'margin-right'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'margin-top'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'max-height'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'max-width'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'min-height'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'min-width'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'opacity'</td><td>as <a href="#animtype-number">number</a></td>

        </tr>

        <tr>

          <td>'outline-color'</td><td>as <a href="#animtype-color">color</a></td>

        </tr>

        <tr>

          <td>'outline-width'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'padding-bottom'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'padding-left'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'padding-right'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'padding-top'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'right'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'text-indent'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'text-shadow'</td><td>as <a href="#animtype-shadow-list">shadow list</a></td>

        </tr>

        <tr>

          <td>'top'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'vertical-align'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'visibility'</td><td>as <a href="#animtype-visibility">visibility</a></td>

        </tr>

        <tr>

          <td>'width'</td><td>as <a href="#animtype-lpcalc">length, percentage, or calc</a></td>

        </tr>

        <tr>

          <td>'word-spacing'</td><td>as <a href="#animtype-length">length</a></td>

        </tr>

        <tr>

          <td>'z-index'</td><td>as <a href="#animtype-integer">integer</a></td>

        </tr>

      </table>

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

 ----------------------------------------------------------------------------

      <p>

        All properties defined as animatable in the SVG specification, provided

        they are one of the property types listed above.

       </p>

      <!-- <table>

        <tr>

          <th>Property Name</th><th>Type</th>

        </tr>

        <tr>

          <td>stop-color</td><td>as <a href="#animtype-color">color</a></td>

        </tr>

        <tr>

          <td>stop-opacity</td><td>as <a href="#animtype-number">number</a></td>

        </tr>

        <tr>

          <td>fill</td><td>as <a href="#animtype-paintserver">paint server</a></td>

        </tr>

        <tr>

          <td>fill-opacity</td><td>as <a href="#animtype-number">number</a></td>

        </tr>

        <tr>

          <td>stroke</td><td>as <a href="#animtype-paintserver">paint server</a></td>

        </tr>

        <tr>

          <td>stroke-dasharray</td><td>as <a href="#animtype-repeatable-list">repeatable list</a> of <a href="#animtype-number">number</a></td>

        </tr>

        <tr>

          <td>stroke-dashoffset</td><td>as <a href="#animtype-number">number</a></td>

        </tr>

        <tr>

          <td>stroke-miterlimit</td><td>as <a href="#animtype-number">number</a></td>

        </tr>

        <tr>

          <td>stroke-opacity</td><td>as <a href="#animtype-number">number</a></td>

        </tr>

        <tr>

          <td>stroke-width</td><td>as <a href="#animtype-number">number</a></td>

        </tr>

        <tr>

          <td>viewport-fill</td><td>as <a href="#animtype-color">color</a></td>

        </tr>

        <tr>

          <td>viewport-fill-opacity</td><td>as <a href="#animtype-color">color</a></td>

        </tr>

       </table> -->

 Security Considerations {#security}

 ===================================

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

 The security implications of this specification are limited

 because it doesn't allow Web content to do things

 that it could not do before.

 Rather, it allows things that could previously be done with script

 to be done declaratively,

 and it ways that implementations can optimize (for frame rate and

 CPU usage).

 One of the major categories of optimizations

 that implementations can make is implementing animation

 of certain high-value properties (such as 'transform' and 'opacity')

 run on a browser's compositor thread or process

 without updating style or layout on the main Web content thread

 unless up-to-date style data are needed.

 This optimization often requires allocations of graphics memory

 to display the contents of the element being animated.

 Implementations should take care to ensure that Web content

 cannot trigger unsafe out-of-memory handling

 by using large numbers of animations

 or animations on elements covering large areas

 (where large may be defined in terms of pre-transform or post-transform size).

 Privacy Considerations {#privacy}

 =================================

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

 As for security, the privacy considerations of this specification are limited

 because it does not allow Web content to do things that it could not do before.

 This specification may provide additional mechanisms that help to determine

 characteristics of the user's hardware or software.

 However, ability to determine performance characteristics of the user's

 hardware or software is common to many Web technologies,

 and this specification does not introduce new capabilities.

 As described in [[#accessibility]],

 implementations may provide mitigations to help users with disabilities.

 These mitigations are likely to be detectable by Web content,

 which means that users who would benefit from these mitigations

 may face a tradeoff between keeping their disability private from

 the Web content or benefiting from the mitigation.

 Accessibility Considerations {#accessibility}

 =============================================

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

 Motion {#accessibility-motion}

 ------------------------------

 This specification provides declarative mechanisms for animations

 that previously needed to be done using script.

 Providing a declarative mechanism has multiple effects:

 it makes such animations easier to make and thus likely to be more common,

 but it also makes it easier for user agents to modify those animations

 if such modifications are needed to meet a user's accessibility needs.

 Thus, users who are sensitive to movement,

 or who require additional time to read or understand content,

 may benefit from user agent features that allow

 animations to be disabled or slowed down.

 (But see [[#privacy]] for information on the privacy implications

 of such mitigations.)

 User agent implementors should be aware that Web content

 may depend on the firing of <a href="#transition-events">transition events</a>,

 so implementations of such mitigations may wish to fire transition events

 even if the transitions were not run as continuous animations.

 However, it is probably poor practice for Web content to depend on

 such events to function correctly.

 Cascade {#accessibility-cascade}

 --------------------------------

 The CSS <a>cascade</a> is a general mechanism in CSS

 that allows user needs to interact with author styles.

 This specification interacts with the cascade,

 but since it only allows animation between values

 that result from the existing cascade rules,

 it does not interfere with the user's ability to force

 CSS properties to have particular values.

 The cascade also allows users to disable transitions entirely

 by overriding the transition properties.

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

 ==========================================================

 <p>The following are the substantive changes made since the

 <a href="https://www.w3.org/TR/2013/WD-css3-transitions-20131119/">Working Draft

 dated 19 November 2013</a>:</p>

 <ul>

   <li>Values that cannot be interpolated are transitioned when the timing function crosses its midpoint, instead of not running transitions and changing immediately.</li>

   <li>Canceling and interrupting of running transitions is defined much more precisely.  This includes the after-change style no longer including styles from CSS Transitions.</li>

   <li>Completion of transitions is defined somewhat more precisely.</li>

   <li>The transitionend event is no longer cancelable.  This is since it has no default action, so canceling it would have no meaning.  It also matches the animation events.</li>

   <li>The {{transitionrun}}, {{transitionstart}}, and {{transitioncancel}}

   events have been added.</li>

   <li>The interpolation of ''shadow/inset'' values on shadow lists is no longer backwards.</li>

   <li>A [[#conformance]] section, [[#security]] section, [[#privacy]] section, [[#accessibility]] section, and [[#idl-index]] have been added</li>

   <li>The identifiers accepted by 'transition-property' are defined in terms of <<custom-ident>>.</li>

   <li>Define a little bit more about when changes to computed values happen, by saying at least that implementations must not update the effects of computed values without actually updating computed values.</li>

 </ul>

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

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

 <ol>

   <li>see the <a href="https://www.w3.org/TR/2013/WD-css3-transitions-20131119/#changes">changes section in the 19 November 2013 Working Draft</a>

   <li>see the <a href="https://www.w3.org/TR/2013/WD-css3-transitions-20130212/ChangeLog">the ChangeLog</a> for changes in previous working drafts

   <li>For more details on these changes, see the version control change logs, which are split in three parts because of file renaming: <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.bs">change log since 2015 January 26</a>, <a href="https://hg.csswg.org/drafts/log/tip/css-transitions/Overview.src.html">change log from 2013 March 28 to 2015 January 26</a>, <a href="https://hg.csswg.org/drafts/log/tip/css3-transitions/Overview.src.html">change log before 2013 March 28</a>.

 </ol>

 Acknowledgments {#acknowledgments}

 ==================================

 <p>Thanks especially to the feedback from

 Tab Atkins,

 Carine Bournez,

 Aryeh Gregor,

 Vincent Hardy,

 Anne van Kesteren,

 Cameron McCormack,

 Alex Mogilevsky,

 Jasper St. Pierre,

 Estelle Weyl,

 and all the rest of the

 <a href="http://lists.w3.org/Archives/Public/www-style/">www-style</a> community.</p>