Overview.bs

<h1>CSS Animations Level 1</h1>

<pre class='metadata'>
Status: ED
Work Status: Refining
Shortname: css-animations
Level: 1
Group: csswg
TR: http://www.w3.org/TR/css3-animations/
ED: http://dev.w3.org/csswg/css-animations/
Previous Version: http://www.w3.org/TR/2013/WD-css3-animations-20130219/
Previous Version: http://www.w3.org/TR/2012/WD-css3-animations-20120403/
Editor: Dean Jackson, Apple Inc., dino@apple.com
Editor: L. David Baron, Mozilla, http://dbaron.org/
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/
Editor: Brian Birtles, Mozilla Japan, bbirtles@mozilla.com
Former Editor: David Hyatt, Apple Inc.
Former Editor: Chris Marrin, Apple Inc.
Former Editor: Sylvain Galineau, Adobe, galineau@adobe.com
!Issues List: <a href="https://www.w3.org/Bugs/Public/buglist.cgi?component=Animations&list_id=36653&product=CSS&query_format=advanced&resolution=---">In Bugzilla</a>

Abstract: This CSS module describes a way for authors to animate the values of CSS properties over time, using keyframes. The behavior of these keyframe animations can be controlled by specifying their duration, number of repeats, and repeating behavior.

At Risk: the <code>pseudoElement</code> property of the ''AnimationEvent'' interface

Link Defaults: css-values-3 (type) <time>, cssom-1 (interface) cssstyledeclaration, dom-core-ls (interface) event, webidl (type) SyntaxError, css-position-3 (property) left
Ignored Terms: domstring, float, animationeventinit, event, eventinit, eventtarget, document
</pre>
<pre class="anchors">
url: http://w3c.github.io/dom/#constructing-events; type: dfn; text: event constructor;
</pre>
<pre class="link-defaults">
spec:cssom-1; type:interface; text:CSSRule
</pre>

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

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

	CSS Transitions [[CSS3-TRANSITIONS]] provide a way to interpolate
	CSS property values when they change as a result of underlying
	property changes. This provides an easy way to do simple animation,
	but the start and end states of the animation are controlled by the
	existing property values, and transitions provide little control to
	the author on how the animation progresses.

	This proposal introduces defined animations, in which the author can
	specify the changes in CSS properties over time as a set of keyframes.
	Animations are similar to transitions in that they change the
	presentational value of CSS properties over time. The principal difference
	is that while transitions trigger implicitly when property values change,
	animations are explicitly executed when the animation properties are applied.
	Because of this, animations require explicit values for the properties
	being animated. These values are specified using animation keyframes,
	described below.

	Many aspects of the animation can be controlled, including how many times
	the animation iterates, whether or not it alternates between the begin and
	end values, and whether or not the animation should be running or paused.
	An animation can also delay its start time.

<h2 id="values">
Values</h2>

	This specification follows the CSS property definition conventions
	from [[!CSS21]]. Value types not defined in this specification are
	defined in CSS Level 2 Revision 1 [[!CSS21]]. Other CSS modules may
	expand the definitions of these value types: for example [[!CSS3VAL]],
	when combined with this module, expands the definition of the
	<<length>> value type as used in this specification.

	In addition to the property-specific values listed in their definitions,
	all properties defined in this specification also accept the ‘initial’
	and ‘inherit’ keyword as their property value. For readability it has
	not been repeated explicitly.

<h2 id="animations">
Animations</h2>

	CSS Animations affect computed property values. This effect happens by
	adding a specified value to the CSS cascade ([[!CSS3CASCADE]]) (at the
	level for CSS Animations) that will produce the correct computed value
	for the current state of the animation. As defined in [[!CSS3CASCADE]],
	animations override all normal rules, but are overridden by !important
	rules.

	If at one point in time there are multiple animations specifying behavior
	for the same property, the animation which occurs last in the value
	of 'animation-name' will override the other animations at that point.

	An animation does not affect the computed value before the application of
	the animation, before the animation delay has expired, and after the end of
	the animation.

	While running, the animation computes the value of those properties
	it animates. Other values may take precedence over the animated value
	according to the CSS cascade ([[!CSS3CASCADE]]).

	The start time of an animation is the time at which the style applying
	the animation and the corresponding @keyframes rule are both resolved.
	If an animation is specified for an element but the corresponding
	@keyframes rule does not yet exist, the animation cannot start; the
	animation will start from the beginning as soon as a matching @keyframes
	rule can be resolved. An animation specified by dynamically modifying the
	element's style will start when this style is resolved; that may be
	immediately in the case of a pseudo style rule such as hover, or may be
	when the scripting engine returns control to the browser (in the case of
	style applied by script). Note that dynamically updating keyframe style
	rules does not start or restart an animation.


	An animation applies to an element if its name appears as one of the
	identifiers in the computed value of the 'animation-name' property and the
	animation uses a valid @keyframes rule. Once an
	animation has started it continues until it ends or the 'animation-name' is
	removed. Changing the values of animation properties while the animation
	is running has no effect on the amount of time that has elapsed since the
	animation started running i.e. once the animation is running, updates to
	'animation-delay' have no effect. The remainder of the animation runs
	according to the new animation property values.

	Note also that changing the value
	of 'animation-name' does not necessarily restart an animation (e.g., if a list
	of animations are applied and one is removed from the list, only that animation
	will stop; The other animations will continue). In order to restart an animation,
	it must be removed then reapplied.

	The end of the animation is defined by the combination of the
	'animation-duration', 'animation-iteration-count' and 'animation-fill-mode'
	properties.

	<div class='example'>
		<pre>
		div {
		  animation-name: diagonal-slide;
		  animation-duration: 5s;
		  animation-iteration-count: 10;
		}

		@keyframes diagonal-slide {

		  from {
		    left: 0;
		    top: 0;
		  }

		  to {
		    left: 100px;
		    top: 100px;
		  }

		}
		</pre>

		This will produce an animation that moves an element from (0, 0) to
		(100px, 100px) over five seconds and repeats itself nine times
		(for a total of ten iterations).

	</div>

	Setting the 'display' property to ''display/none'' will terminate any running animation applied
	to the element and its descendants. If an element has a 'display' of ''display/none'', updating
	'display' to a value other than ''display/none'' will start all animations applied to the element
	by the 'animation-name' property, as well as all animations applied to descendants
	with 'display' other than ''display/none''.

	While authors can use animations 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 Guideline 2.3: Seizures: Do not design content
	in a way that is known to cause seizures ([[!WCAG20]]).

	Implementations may ignore animations when the rendering medium is not interactive e.g. when printed.
	A future version of this specification may define how to render animations for these media.

<h2 id="keyframes">
Keyframes</h2>

	Keyframes are used to specify the values for the animating properties at various points
	during the animation. The keyframes specify the behavior of one cycle of the animation;
	the animation may iterate one or more times.

	Keyframes are specified using a specialized CSS at-rule. A <dfn>@keyframes</dfn> rule consists of the
	keyword "@keyframes", followed by an identifier giving a name for the animation (which will
	be referenced using 'animation-name'), followed by a set of style rules (delimited by curly
	braces).

	The keyframe selector for a keyframe style rule consists of a comma-separated list of
	percentage values or the keywords ''from'' or ''to''. The selector is used to specify the
	percentage along the duration of the animation that the keyframe represents. The keyframe
	itself is specified by the block of property values declared on the selector. The keyword
	''from'' is equivalent to the value ''0%''. The keyword ''to'' is equivalent to the value ''100%''.
	<span class="note">Note that the percentage unit specifier must be used on percentage values.
	Therefore, ''0'' is an invalid keyframe selector.</span>

	If a ''0%'' or ''from'' keyframe is not specified, then the user agent constructs a ''0%'' keyframe
	using the computed values of the properties being animated. If a ''100%'' or ''to'' keyframe is not
	specified, then the user agent constructs a ''100%'' keyframe using the computed values of the
	properties being animated. If a keyframe selector specifies negative percentage values or values
	higher than ''100%'', then the keyframe will be ignored.

	The keyframe declaration block for a keyframe rule consists of properties and values. The properties
	defined by this specification are ignored in these rules, with the exception of
	'animation-timing-function', the behavior of which is described below. In addition, keyframe rule
	declarations qualified with !important are ignored.

	The @keyframes rule that is used by an animation will be the last one encountered in sorted rules
	order that matches the name of the animation specified by the 'animation-name' property.

	<div class='example'>
		<pre>
		div {
		  animation-name: slide-right;
		  animation-duration: 2s;
		}

		@keyframes slide-right {

		  from {
		    margin-left: 0px;
		  }

		  50% {
		  	margin-left: 110px;
		  	opacity: 1;
		  }

		  50% {
		  	 opacity: 0.9;
		  }

		  to {
		    margin-left: 200px;
		  }

		}
		</pre>

		At the 1s mark, the slide-right animation will have the same state as if we had defined the 50% rule like this:

		<pre>

		@keyframes slide-right {

		  50% {
		  	margin-left: 110px;
		  	opacity: 0.9;
		  }

		  to {
		    margin-left: 200px;
		  }

		}
		</pre>

	</div>

	To determine the set of keyframes, all of the values in the selectors are sorted in increasing order
	by time. The rules within the @keyframes rule then cascade; the properties of a keyframe may thus derive
	from more than one @keyframes rule with the same selector value.

	If a property is not specified for a keyframe, or is specified but invalid, the animation of that
	property proceeds as if that keyframe did not exist. Conceptually, it is as if a set of keyframes is
	constructed for each property that is present in any of the keyframes, and an animation is run
	independently for each property.

	<div class='example'>
		<pre>
		@keyframes wobble {
		  0% {
		    left: 100px;
		  }

		  40% {
		    left: 150px;
		  }

		  60% {
		    left: 75px;
		  }

		  100% {
		    left: 100px;
		  }
		}
		</pre>

		Four keyframes are specified for the animation named "wobble". In the first keyframe,
		shown at the beginning of the animation cycle, the value of the 'left' property being
		animated is ''100px''. By 40% of the animation duration, 'left' has animated to ''150px''.
		At 60% of the animation duration, 'left' has animated back to ''75px''. At the end of the
		animation cycle, the value of 'left' has returned to ''100px''. The diagram below shows
		the state of the animation if it were given a duration of ''10s''.

		<figure>
			<img src="./animation1.png" alt="">
			<figcaption>Animation states specified by keyframes</figcaption>
		</figure>
	</div>

	The following is the grammar for the keyframes rule:

	<pre>
		keyframes_rule: KEYFRAMES_SYM S+ IDENT S* '{' S* keyframes_blocks '}' S*;

		keyframes_blocks: [ keyframe_selector '{' S* declaration? [ ';' S* declaration? ]* '}' S* ]* ;

		keyframe_selector: [ FROM_SYM | TO_SYM | PERCENTAGE ] S* [ ',' S* [ FROM_SYM | TO_SYM | PERCENTAGE ] S* ]*;

		@{K}{E}{Y}{F}{R}{A}{M}{E}{S}   {return KEYFRAMES_SYM;}
		{F}{R}{O}{M}                   {return FROM_SYM;}
		{T}{O}                         {return TO_SYM;}
	</pre>

	<p class="issue">
		This specification needs to define
		how the value is determined from the keyframes,
		like the section on
		<a href="http://dev.w3.org/csswg/css-transitions/#application">Application of transitions</a>
		does for CSS Transitions.
	</p>

<h3 id="timing-functions">
Timing functions for keyframes</h3>

	A keyframe style rule may also declare the timing function that is to be used as the animation
	moves to the next keyframe.

	<div class='example'>
		<pre>
		@keyframes bounce {

		  from {
		    top: 100px;
		    animation-timing-function: ease-out;
		  }

		  25% {
		    top: 50px;
		    animation-timing-function: ease-in;
		  }

		  50% {
		    top: 100px;
		    animation-timing-function: ease-out;
		  }

		  75% {
		    top: 75px;
		    animation-timing-function: ease-in;
		  }

		  to {
		    top: 100px;
		  }

		}
		</pre>

		Five keyframes are specified for the animation named "bounce". Between the first and second
		keyframe (i.e., between 0% and 25%) an ease-out timing function is used. Between the second
		and third keyframe (i.e., between 25% and 50%) an ease-in timing function is used. And so on.
		The effect will appear as an element that moves up the page 50px, slowing down as it reaches
		its highest point then speeding up as it falls back to 100px. The second half of the animation
		behaves in a similar manner, but only moves the element 25px up the page.
	</div>

	A timing function specified on the ''to'' or ''100%'' keyframe is ignored.

	See the 'animation-timing-function' property for more information.

<h3 id="animation-name">
The 'animation-name' property</h3>

	 The 'animation-name' property defines a list of animations that apply. Each name is used to select
	 the keyframe at-rule that provides the property values for the animation. If the name does not match
	 any keyframe at-rule, there are no properties to be animated and the animation will not execute.
	 Furthermore, if the animation name is <code>none</code> then there will be no animation. This can be
	 used to override any animations coming from the cascade. If multiple animations are attempting to
	 modify the same property, then the animation closest to the end of the list of names wins.

	Each animation listed by name should have a corresponding value for the other animation properties
	listed below. If the lists of values for the other animation properties do not have the same length,
	the length of the 'animation-name' list determines the number of items in each list examined when
	starting animations. 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 'animation-name', 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.

	Note: This is analogous to the behavior of the ‘background-*’properties, with ‘background-image’ analogous
	to 'animation-name'.

	<pre class='propdef'>
	Name: animation-name
	Value: <<single-animation-name>>#
	Initial: none
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: none
	Animatable: no
	Percentages: N/A
	Media: interactive
	Computed value: As specified
	Canonical order: per grammar
	</pre>

	<dfn>&lt;single-animation-name></dfn> = none | <<custom-ident>>

	The values of 'animation-name' have the following meanings:

	<dl dfn-type=value dfn-for=animation-name>
		<dt><dfn>none</dfn>
		<dd>
			No keyframes are specified at all, so there will be no animation.
			Any other animations properties specified for this animation have no effect.

		<dt><dfn><<custom-ident>></dfn>
		<dd>
			The animation will use the keyframes with the name specified by the <<custom-ident>>,
			if they exist.If no such keyframes exist, there is no animation.

			The <a href="http://www.w3.org/TR/css3-values/#common-keywords">CSS-wide keywords</a>
			and keywords defined by this property are not valid animation names.
	</dl>

<h3 id="animation-duration">
The 'animation-duration' property</h3>

	The 'animation-duration' property defines duration of a single animation cycle.

	<pre class='propdef'>
	Name: animation-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: As specified
	Canonical order: per grammar
	</pre>

	<dl>
		<dt><dfn value for=animation-duration><<time>></dfn>
		<dd>
			The <<time>> specifies the length of time that an animation takes to complete one cycle.
			A negative <<time>> is invalid.

			If the <<time>> is ''0s'', like the initial value,
			the keyframes of the animation have no effect,
			but the animation itself still occurs instantaneously.
			Specifically, start and end events are fired; if
			'animation-fill-mode' is set to ''backwards'' or
			''both'', the first frame of the animation, as defined by
			'animation-direction', will be displayed during the
			'animation-delay'. Then the last frame of the animation,
			as defined by 'animation-direction', will be displayed if
			'animation-fill-mode' is set to ''forwards'' or ''both''.
			If 'animation-fill-mode' is set to ''animation-fill-mode/none''
			then the animation has no visible effect.
	</dl>

<h3 id="animation-timing-function">
The 'animation-timing-function' property</h3>

	The 'animation-timing-function' property describes how the animation will progress over
	one cycle of its duration. See the 'transition-timing-function' property [[!CSS3-TRANSITIONS]]
	for a complete description of timing function calculation.

	<pre class='propdef'>
	Name: animation-timing-function
	Value: <<single-timing-function>>#
	Initial: ease
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: interactive
	Computed value: As specified
	Canonical order: per grammar
	</pre>

	The values and meaning of <dfn>&lt;single-timing-function></dfn>
	are identical to those of <<single-transition-timing-function>> [[!CSS3-TRANSITIONS]].

	The timing function specified applies to each iteration of the animation,
	not the entire animation in full.
	For example, if an animation has ''animation-timing-function: ease-in-out; animation-iteration-count: 2;'',
	it will ease in at the start,
	ease out as it approaches the end of its first iteration,
	ease in at the start of its second iteration,
	and ease out again as it approaches the end of the animation.

	When specified in a keyframe,
	'animation-timing-function' defines
	the progression of the animation
	between the current keyframe
	and the next keyframe for the animating property
	in sorted keyframe selector order
	(which may be an implicit 100% keyframe).
	The specified timing function
	will apply over this interval
	independently of the animation's current direction.

<h3 id="animation-iteration-count">
The 'animation-iteration-count' property</h3>

	The 'animation-iteration-count' property specifies the number of times an animation cycle
	is played. The initial value is ''1'', meaning the animation will play from beginning to end
	once. This property is often used in conjunction with an
	'animation-direction' value of ''alternate'', which will cause the animation to play in
	reverse on alternate cycles.

	The time window during which the animation is active
	(<code>duration</code> x <code>iteration-count</code>)
	is known as the <dfn>active duration</dfn>.

	<pre class='propdef'>
	Name: animation-iteration-count
	Value: <<single-animation-iteration-count>>#
	Initial: 1
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: interactive
	Computed value: As specified
	Canonical order: per grammar
	</pre>

	<dfn>&lt;single-animation-iteration-count></dfn> = infinite | <<number>>

	<dl dfn-type=value dfn-for=animation-iteration-count>
		<dt><dfn>infinite</dfn>
		<dd>
			The animation will repeat forever.

		<dt><dfn><<number>></dfn>
		<dd>
			<p>The animation will repeat the specified number of times.
			If the number is not an integer,
			the animation will end partway through its last cycle.
			Negative numbers are invalid.

			<p>A value of ''0'' is valid and, similar to an 'animation-duration'
			of ''0s'', causes the animation to occur instantaneously.
	</dl>

	If the animation has a duration of ''0s'', it will occur instantaneously for any
	valid value of 'animation-iteration-count', including ''infinite''.


<h3 id="animation-direction">
The 'animation-direction' property</h3>

	The 'animation-direction' property defines whether or not the animation should play in reverse
	on some or all cycles. When an animation is played in reverse the timing functions are also
	reversed. For example, when played in reverse an ''ease-in'' animation would appear to be an
	''ease-out'' animation.

	<pre class='propdef'>
	Name: animation-direction
	Value: <<single-animation-direction>>#
	Initial: normal
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: interactive
	Computed value: As specified
	Canonical order: per grammar
	</pre>

	<dfn>&lt;single-animation-direction></dfn> = normal | reverse | alternate | alternate-reverse

	<dl dfn-type=value dfn-for=animation-direction>
		<dt><dfn>normal</dfn>
		<dd>
			All iterations of the animation are played as specified.

		<dt><dfn>reverse</dfn>
		<dd>
			All iterations of the animation are played in the reverse direction
			from the way they were specified.

		<dt><dfn>alternate</dfn>
		<dd>
			The animation cycle iterations that are odd counts are played in the
			normal direction, and the animation cycle iterations that are even
			counts are played in a reverse direction.

		<dt><dfn>alternate-reverse</dfn>
		<dd>
			The animation cycle iterations that are odd counts are played in the
			reverse direction, and the animation cycle iterations that are even
			counts are played in a normal direction.
	</dl>

	Note: For the purpose of determining whether an iteration is even or odd,
	iterations start counting from 1.


<h3 id="animation-play-state">
The 'animation-play-state' property</h3>

	The 'animation-play-state' property defines whether the animation is running or paused.

	<pre class='propdef'>
	Name: animation-play-state
	Value: <<single-animation-play-state>>#
	Initial: running
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: interactive
	Computed value: As specified
	Canonical order: per grammar
	</pre>

	<dfn>&lt;single-animation-play-state></dfn> = running | paused

	<dl dfn-type=value dfn-for=animation-play-state>
		<dt><dfn>running</dfn>
		<dd>
			While this property is set to ''running'',
			the animation proceeds as normal.

		<dt><dfn>paused</dfn>
		<dd>
			While this property is set to ''paused'',
			the animation is paused.
			The animation continues to apply to the element with the progress it had made before being paused.
			When unpaused (set back to ''running''), it restarts from where it left off,
			as if the "clock" that controls the animation had stopped and started again.

			If the property is set to ''paused'' during the delay phase of the animation,
			the delay clock is also paused and resumes as soon as 'animation-play-state' is set back to ''running''.
	</dl>

<h3 id="animation-delay">
The 'animation-delay' property</h3>

	The 'animation-delay' property defines when the animation will start. It allows an animation
	to begin execution some time after it is applied,
	or to appear to have begun execution some time <em>before</em> it is applied.

	<pre class='propdef'>
	Name: animation-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: As specified
	Canonical order: per grammar
	</pre>

	<dl>
		<dt><dfn value for=animation-delay><<time>></dfn>
		<dd>
			The <<time>> defines how long of a delay there is between the start of the animation
			(when the animation is applied to the element via these properties)
			and when it begins executing.
			A delay of ''0s'' (the initial value) means that the animation will execute as soon as it is applied.

			A negative delay is <strong>valid</strong>.
			Similar to a delay of ''0s'', it means that the animation executes immediately,
			but is automatically progressed by the absolute value of the delay,
			as if the animation had started the specified time in the past,
			and so it appears to start partway through its
			<a href="#animation-iteration-count">active duration</a>.
			If an animation's keyframes have an implied starting value,
			the values are taken from the time the animation starts,
			not some time in the past.
	</dl>

<h3 id="animation-fill-mode">
The 'animation-fill-mode' property</h3>

	The 'animation-fill-mode' property defines what values are applied by the animation
	outside the  time it is executing. By default, an animation will not affect property
	values between the time it is applied (the ‘animation-name’ property is set on an
	element) and the time it begins execution (which is determined by the 'animation-delay'
	property). Also, by default an animation does not affect property values after the
	animation ends (determined by the 'animation-duration' and 'animation-iteration-count' properties).
	The 'animation-fill-mode' property can override this behavior. Dynamic updates to the property will
	be reflected by property values as needed, whether during the animation delay or after the animation ends.

	<pre class='propdef'>
	Name: animation-fill-mode
	Value: <<single-animation-fill-mode>>#
	Initial: none
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: interactive
	Computed value: As specified
	Canonical order: per grammar
	</pre>

	<dfn>&lt;single-animation-fill-mode></dfn> = none | forwards | backwards | both

	<dl dfn-type=value dfn-for=animation-fill-mode>
		<dt><dfn>none</dfn>
		<dd>
			The animation has no effect when it is applied but not executing.

		<dt><dfn>forwards</dfn>
		<dd>
			After the animation ends (as determined by its 'animation-iteration-count'), the animation
			will apply the property values for the time the animation ended. When 'animation-iteration-count'
			is an integer greater than zero, the values applied will be those for the end of the last
			completed iteration of the animation (rather than the values for the start of the iteration
			that would be next). When 'animation-iteration-count' is zero, the values applied will be
			those that would start the first iteration (just as when 'animation-fill-mode' is ''backwards'').

		<dt><dfn>backwards</dfn>
		<dd>
			During the period defined by 'animation-delay', the animation will apply the property values
			defined in the keyframe that will start the first iteration of the animation.
			These are either the values of the ''from'' keyframe (when 'animation-direction' is ''normal''
			or ''alternate'') or those of the ''to'' keyframe (when 'animation-direction' is ''reverse''
			or ''alternate-reverse'').

		<dt><dfn>both</dfn>
		<dd>
			The effects of both ''forwards'' and ''backwards'' fill apply.
	</dl>

<h3 id="animation">
The 'animation' shorthand property</h3>

	The 'animation' shorthand property is a comma-separated list of animation definitions. Each item in
	the list gives one item of the value for all of the subproperties of the shorthand, which are known
	as the animation properties. (See the definition of 'animation-name' for what happens when these
	properties have lists of different lengths, a problem that cannot occur when they are defined using
	only the 'animation' shorthand.)

	<pre class='propdef'>
	Name: animation
	Value: 	<<single-animation>>#
	Initial: see individual properties
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: interactive
	Computed value: As specified
	Canonical order: per grammar
	</pre>

	<dfn>&lt;single-animation></dfn> = <<time>> || <<single-timing-function>> || <<time>> || <<single-animation-iteration-count>> || <<single-animation-direction>> || <<single-animation-fill-mode>> || <<single-animation-play-state>> || <<single-animation-name>>

	Note that order is important within each animation definition: the first value in each
	<<single-animation>> that can be parsed as a <<time>> is assigned to the 'animation-duration',
	and the second value in each <<single-animation>> that can be parsed as a <<time>> is assigned to
	'animation-delay'.

	Note that order is also important within each animation definition for distinguishing
	<<single-animation-name>> values from other keywords. When parsing, keywords that are valid for
	properties other than 'animation-name'
	whose values were not found earlier in the shorthand
	must be accepted for those properties rather than for
	'animation-name'. Furthermore, when serializing, default values of other properties must be
	output in at least the cases necessary to distinguish an 'animation-name' that could
	be a value of another property, and may be output in additional cases.

	<div class="example">
		For example, a value parsed from ''animation: 3s none backwards''
		(where 'animation-fill-mode' is ''animation-fill-mode/none''
		and 'animation-name' is ''animation-name/backwards'')
		must not be serialized as ''animation: 3s backwards''
		(where 'animation-fill-mode' is ''animation-fill-mode/backwards''
		and 'animation-name' is ''animation-name/none'').
	</div>

<h2 id="events">
Animation Events</h2>

	Several animation-related events are available through the DOM Event system. The start and
	end of an animation, and the end of each iteration of an animation, all generate DOM events.
	An element can have multiple properties being animated simultaneously. This can occur either
	with a single 'animation-name' value with keyframes containing multiple properties, or with
	multiple 'animation-name' values. For the purposes of events, each 'animation-name' specifies
	a single animation. Therefore an event will be generated for each 'animation-name' value and
	not necessarily for each property being animated.

	Any animation for which a valid keyframe rule is defined will run
	and generate events; this includes animations with empty keyframe rules.

	The time the animation has been running is sent with each event generated. This allows the event
	handler to determine the current iteration of a looping animation or the current position of an
	alternating animation. This time does not include any time the animation was in the ''paused''
	play state.

<h3 id="interface-animationevent">
The <code>AnimationEvent</code> Interface</h3>

	The <code>AnimationEvent</code> interface provides specific contextual information associated with
	Animation events.

<h4 id="interface-animationevent-idl">
IDL Definition</h4>

	<pre class="idl">
		[Constructor(DOMString type, optional AnimationEventInit animationEventInitDict)]
		interface AnimationEvent : Event {
		  readonly attribute DOMString animationName;
		  readonly attribute float elapsedTime;
		  readonly attribute DOMString pseudoElement;
		};
		dictionary AnimationEventInit : EventInit {
		  DOMString animationName = "";
		  float elapsedTime = 0.0;
		  DOMString pseudoElement = "";
		};
	</pre>

<h4 id="interface-animationevent-attributes">
Attributes</h4>

	<dl dfn-type=attribute dfn-for=AnimationEvent>
		<dt><dfn>animationName</dfn>, of type <a interface>DOMString</a>, readonly
		<dd>
			The value of the 'animation-name' property of the animation that fired the event.
		<dt><dfn>elapsedTime</dfn>, of type float, readonly
		<dd>
			The amount of time the animation has been running, in seconds, when this event fired,
			excluding any time the animation was paused. For an ''animationstart'' event, the
			elapsedTime is zero unless there was a negative value for 'animation-delay', in which
			case the event will be fired with an elapsedTime of (-1 * delay).
		<dt><dfn>pseudoElement</dfn>, of type <a interface>DOMString</a>, readonly
		<dd>
			The name (beginning with two colons) of the CSS pseudo-element on which the animation
			runs (in which case the target of the event is that pseudo-element's corresponding
			element), or the empty string if the animation runs on an element (which means the
			target of the event is that element).
	</dl>

	<dfn>AnimationEvent(type, animationEventInitDict)</dfn> is an <a>event constructor</a>.

<h3 id="event-animationevent">
Types of <code>AnimationEvent</code></h3>

	The different types of animation events that can occur are:

	<dl dfn-type=event dfn-for=animationevent>
		<dt><dfn>animationstart</dfn>
		<dd>
			The <a idl>animationstart</a> event occurs at the start of the animation.
			If there is an 'animation-delay' then this event will fire once the delay
			period has expired. A negative delay will cause the event to fire with
			an elapsedTime equal to the absolute value of the delay; in this case the
			event will fire whether 'animation-play-state' is set to ''running'' or ''paused''.
			<ul>
				<li>Bubbles: Yes</li>
				<li>Cancelable: No</li>
				<li>Context Info: animationName, pseudoElement</li>
			</ul>

		<dt><dfn>animationend</dfn>
		<dd>
			The <a idl>animationend</a> event occurs when the animation finishes.
			<ul>
				<li>Bubbles: Yes</li>
				<li>Cancelable: No</li>
				<li>Context Info: animationName, elapsedTime, pseudoElement</li>
			</ul>

		<dt><dfn>animationiteration</dfn>
		<dd>
			The <a idl>animationiteration</a> event occurs at the end of each iteration of an
			animation, except when an animationend event would fire at the same time.
			This means that this event does not occur for animations with an iteration
			count of one or less.
			<ul>
				<li>Bubbles: Yes</li>
				<li>Cancelable: No</li>
				<li>Context Info: animationName, elapsedTime, pseudoElement</li>
			</ul>
	</dl>

<h2 id="interface-dom">
DOM Interfaces</h2>

	CSS animations are exposed to the CSSOM through a pair of new interfaces describing the keyframes.

    Note: the interfaces defined below reflect the interoperable API available as of this level of the specification. Future levels may deprecate parts of this API and extend others.

<h3 id="interface-cssrule">
The <code>CSSRule</code> Interface</h3>

	The following two rule types are added to the <a idl>CSSRule</a> interface. They provide
	identification for the new keyframe and keyframes rules.

<h4 id="interface-cssrule-idl">
IDL Definition</h4>

	<pre class="idl">
	partial interface CSSRule {
		  const unsigned short KEYFRAMES_RULE = 7;
		  const unsigned short KEYFRAME_RULE = 8;
	};
	</pre>

<h3 id="interface-csskeyframerule">
The <code>CSSKeyframeRule</code> Interface</h3>

	The <a idl>CSSKeyframeRule</a> interface represents the style rule for a single key.

<h4 id="interface-csskeyframerule-idl">
IDL Definition</h4>

	<pre class="idl">
	interface CSSKeyframeRule : CSSRule {
	           attribute DOMString           keyText;
	  readonly attribute CSSStyleDeclaration style;
	};
	</pre>

<h4 id="interface-csskeyframerule-attributes">
Attributes</h4>

	<dl dfn-type=attribute dfn-for=CSSKeyframeRule>

		<dt><dfn attribute for="CSSKeyframeRule">keyText</dfn>
		<dd>
			This attribute represents the keyframe selector as a comma-separated list of
			percentage values. The ''from'' and ''to'' keywords map to 0% and 100%,
			respectively.

            If <a attribute for="CSSKeyframeRule">keyText</a> is updated with an invalid keyframe selector,
            a <a spec="webidl">SyntaxError</a> exception must be thrown.

		<dt><dfn>style</dfn>
		<dd>
			This attribute represents the style associated with this keyframe.
	</dl>

<h3 id="interface-csskeyframesrule">
The <code>CSSKeyframesRule</code> Interface</h3>

	The <a idl>CSSKeyframesRule</a> interface represents a complete set of keyframes for
	a single animation.

<h4 id="interface-csskeyframesrule-idl">
IDL Definition</h4>

	<pre class="idl">
	interface CSSKeyframesRule : CSSRule {
	           attribute DOMString   name;
	  readonly attribute CSSRuleList cssRules;

	  void            appendRule(DOMString rule);
	  void            deleteRule(DOMString select);
	  CSSKeyframeRule? findRule(DOMString select);
	};
	</pre>

<h4 id="interface-csskeyframesrule-attributes">
Attributes</h4>

	<dl dfn-type=attribute dfn-for=CSSKeyframesRule>

		<dt><dfn>name</dfn>
		<dd>
			This attribute is the name of the keyframes, used by the 'animation-name' property.

			Setting this property to a value matching a
			<a href="http://www.w3.org/TR/css3-values/#common-keywords">CSS-wide keyword</a>
			or any keyword defined for the 'animation-name' property will throw a <a spec="webidl">SyntaxError</a>
			exception.

		<dt><dfn>cssRules</dfn>
		<dd>
			This attribute gives access to the keyframes in the list.
	</dl>

<h4 id="interface-csskeyframesrule-appendrule">
The <code>appendRule</code> method</h4>

	The <dfn method for="CSSKeyframesRule">appendRule()</dfn> method appends the passed
	<a idl>CSSKeyframeRule</a> at the end of the keyframes rule.

	Parameters:

	<dl>

		<dt><dfn argument for="CSSKeyframesRule/appendRule(rule)">rule</dfn> of type <a interface>DOMString</a>
		<dd>
			The rule to be appended, expressed in the same syntax as one entry in the
			''@keyframes'' rule. A valid rule is always appended e.g. even if its key(s) already
			exists.
	</dl>

	No Return Value

	No Exceptions

<h4 id="interface-csskeyframesrule-deleterule">
The <code>deleteRule</code> method</h4>

	The <dfn method for="CSSKeyframesRule">deleteRule()</dfn> method deletes the
    last declared <a idl>CSSKeyframeRule</a> matching the specified keyframe selector.
    If no matching rule exists, the method does nothing.

	Parameters:

	<dl>

		<dt><dfn argument for="CSSKeyframesRule/deleteRule(select)">select</dfn> of type <a interface>DOMString</a>
		<dd>
			The keyframe selector of the rule to be deleted: a comma-separated list of percentage values between 0% and 100% or the keywords ''from'' or ''to'' which resolve to 0% and 100%, respectively.

            The number and order of the values in the specified keyframe selector must match those of the targeted keyframe rule(s).  The match is not sensitive to white space around the values in the list.
	</dl>

	No Return Value

	No Exceptions


<h4 id="interface-csskeyframesrule-findrule">
The <code>findRule</code> method</h4>

	The <dfn method for="CSSKeyframesRule">findRule()</dfn> returns the
    last declared <a idl>CSSKeyframeRule</a> matching the specified keyframe selector.
    If no matching rule exists, the method does nothing.

	Parameters:

	<dl>
		<dt><dfn argument for="CSSKeyframesRule/findRule(select)">select</dfn> of type <a interface>DOMString</a>
		<dd>
			The keyframe selector of the rule to be deleted: a comma-separated list of percentage values between 0% and 100% or the keywords ''from'' or ''to'' which resolve to 0% and 100%, respectively.

            The number and order of the values in the specified keyframe selector must match those of the targeted keyframe rule(s). The match is not sensitive to white space around the values in the list.
    </dl>

	Return Value:

	<dl>
		<dt><a idl>CSSKeyframeRule</a>
		<dd>
			The found rule.
	</dl>

	No Exceptions

    <div class='example'>
        For example, given the following animation:
        <pre>

        @keyframes colorful-diagonal-slide {

          from {
            left: 0;
            top: 0;
          }

          10% {
            background-color: blue;
          }

          10% {
            background-color: green;
          }

          25%, 75% {
            background-color: red;
          }

          100% {
            left: 100px;
            top: 100px;
          }

        }
        </pre>

        Assuming the variable <code>anim</code> holds a reference to a CSSKeyframesRule object for this animation, then:

        <pre>
        anim.deleteRule('10%');
        var tenPercent = anim.findRule('10%');
        </pre>

        will start by deleting the last 10% rule i.e. the green background color rule; then find the remaining blue background rule and return it into <code>tenPercent</code>.

        The following:
        <pre>
        var red = anim.findRule('75%');
        </pre>

        will set <code>red</code> to <code>null</code>. The full selector for the red background color rule must be used instead:

        <pre>
        var red = anim.findRule('25%,75%');
        </pre>

        Since ''from'' maps to 0% and ''to'' maps to 100%, we can find these rules using either value:

        <pre>
        var from = anim.findRule('0%'); // Returns from { left: 0; top: 0; } rule
        var to = anim.findRule('to');   // Returns 100% { left: 100px; top: 100px; } rule
        </pre>
    </div>





<h2 id="acknowledgements">
Acknowledgements</h2>

	Thanks especially to the feedback from
	Tab Atkins,
	Brian Birtles,
	Shane Stephens,
	Carine Bournez,
	Christian Budde,
	Anne van Kesteren,
	Øyvind Stenhaug,
	Estelle Weyl,
	and all the rest of the www-style community.

<h2 id="other-open-issues">Other open issues</h2>

<p class="issue">Need to <a href="https://lists.w3.org/Archives/Public/www-style/2015Jul/0391.html">specify how keyframes interact</a>.</p>

<p class="issue">Need to specify better handling of dynamic changes of animation-name; see <a href="https://twitter.com/estellevw/status/617525016104583168">conversation</a>.</p>

<h2 id="wg-resolutions-pending">
Working Group Resolutions that are pending editing</h2>

	<em>This section is informative and temporary.</em>

	The editors are currently behind on editing this spec. The following working group resolutions still
	need to be edited in:

	<ul>
    	<li>2014-09-09 minutes (Antibes f2f)
    		<ul>
    		<li>Detail how/when keyframe values are computed; using <a href="http://lists.w3.org/Archives/Public/www-style/2014Aug/0132.html">G.beta in dbaron's mail</a>
    		<li><del>Agreed that both transitions and animations animate all properties. css-transitions to define animation of non-interoperable/discrete values. They take their starting values below 50% timing function progress, and end values above</del>
    		<li><del>Dynamic changes to animation properties/keyframes. Tab to propose resolution. (<a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=14713">Bug 14713</a>)
    		<li><del>Negative animation-delay values apply against the active duration of the animation i.e. (animation-duration*animation-iteration-count). The delay can thus swallow iterations for which no iteration event will be fired. The start/end events are still fired. Even when delay == (-1*active_duration)
    		<li><del>Fire animation start/end events when animation-duration is zero, with 0 elapsedTime
    		<li><del>If animation-iteration-count is infinite and duration is 0, treat the iteration-count as if it was finite and run a 0s second (option A in <a href="http://lists.w3.org/Archives/Public/www-style/2014Sep/0056.html">Brian's mail</a>)
    		<li><del>If an animation with a negative animation delay is initially paused, the start event still fires
    		</ul>

    	<li>2012-10-29 minutes
	    	<ul>
        	<li><del>Change the animation properties to be dynamically changeable
        	<li><del>@keyframes can be dynamically changed
        	<li><del>When you encounter duplicate animations names, last one wins.</del>
        	<li><del>Make *animations* transition *all* properties. Unless otherwise specified, discrete properties take their starting values below 50% timing function progress, and end values above 50% timing function progress.</del>
        	</ul>
    	<li>2012-12-12 minutes and intermediate comments <del>and 2012-12-19 minutes</del>
    		<ul>
    		<li><del>Animations only run if they contain at least one valid keyframe rule (<a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=15251">Bug</a>)
    		<li><del>When an element changes from display:none to display: non-none, animations start immediately (<a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=14785">Bug</a>)</del>
    		<li><del>An initially-paused animation is still started (fires start events etc.) (<a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=14774">Bug</a>)
    		<li><del>Animations can be paused during their delay phase, which freezes the remaining delay to be applied after it unpauses (<a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=14774">Bug</a>)</del>
    		<li><del>animation-play-state has the same list behavior as the other animation properties, matching the length of animation-name (<a href="https://www.w3.org/Bugs/Public/show_bug.cgi?id=14786">Bug</a>)</del>
    		</ul>
    	<li>2013-02-20 minutes
    		<ul>
    		<li><del>Øyvind's clarification accepted</del>
    		<li><del>keyframe rules cascade</del>
    		<li><del>mark pseudoElement at-risk</del>
    		</ul>
    	<li>2013-05-30 minutes
        	<ul>
        	<li><del>expectations on animations in non-interactive media</del>
        	</ul>
    	<li>2014-01-27 minutes
        	<ul>
        	<li><del>remove text about waiting for document load</del>
        	</ul>