Title: CSS Animations Level 1
Status: WD
Work Status: Refining
Prepare for TR: yes
Date: 2023-03-02
Shortname: css-animations
Level: 1
Group: csswg
TR: https://www.w3.org/TR/css-animations-1/
ED: https://drafts.csswg.org/css-animations/
Previous Version: https://www.w3.org/TR/2018/WD-css-animations-1-20181011/
Previous Version: https://www.w3.org/TR/2017/WD-css-animations-1-20171130/
Previous Version: https://www.w3.org/TR/2013/WD-css3-animations-20130219/
Previous Version: https://www.w3.org/TR/2012/WD-css3-animations-20120403/
Editor: Dean Jackson, Apple Inc., dino@apple.com, w3cid 42080
Editor: L. David Baron, Google https://www.google.com/, https://dbaron.org/, w3cid 15393
Editor: Tab Atkins Jr., Google, http://xanthir.com/contact/, w3cid 42199
Editor: Brian Birtles, Invited Expert, brian@birchill.co.jp, w3cid 43194
Former Editor: David Hyatt, Apple Inc.
Former Editor: Chris Marrin, Apple Inc.
Former Editor: Sylvain Galineau, Adobe, galineau@adobe.com
!Issues List: https://github.com/w3c/csswg-drafts/labels/css-animations-1



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.

Link Defaults: css-values-3 (type) , cssom-1 (interface) cssstyledeclaration, dom-core-ls (interface) event, webidl (type) SyntaxError
Ignored Terms: domstring, animationeventinit, event, eventinit, eventtarget, document

url: https://dom.spec.whatwg.org/#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

spec:cssom-1; type:interface; text:CSSRule

Introduction

This section is not normative 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.

Value Definitions

This specification follows the CSS property definition conventions from [[!CSS2]] using the value definition syntax from [[!CSS-VALUES-3]]. Value types not defined in this specification are defined in CSS Values & Units [[!CSS-VALUES-3]]. Combination with other CSS modules may expand the definitions of these value types. In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

CSS Animations Model

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 some 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 (that is, when the 'animation-name' property is set on an element) or after it is removed. Furthermore, typically an animation does not affect the computed value before the animation delay has expired or after the end of the animation, but may do so depending on the 'animation-fill-mode' property. 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]]). While an animation is applied but has not finished, or has finished but has an 'animation-fill-mode' of ''forwards'' or ''both'', the user agent must act as if the 'will-change' property ([[!css-will-change-1]]) on the element additionally includes all the properties animated by the animation. 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 re-start 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. Changes to the values of animation properties while the animation is running apply as if the animation had those values from when it began. For example, shortening the 'animation-delay' may cause the animation to jump forwards or even finish immediately and dispatch an {{animationend}} event. Conversely, extending the 'animation-delay' may cause an animation to re-start and dispatch an {{animationstart}} event. The same @keyframes rule name may be repeated within an 'animation-name'. Changes to the 'animation-name' update existing animations by iterating over the new list of animations from last to first, and, for each animation, finding the last matching animation in the list of existing animations. If a match is found, the existing animation is updated using the animation properties corresponding to its position in the new list of animations, whilst maintaining its current playback time as described above. The matching animation is removed from the existing list of animations such that it will not match twice. If a match is not found, a new animation is created. As a result, updating 'animation-name' from ‘a’ to ‘a, a’ will cause the existing animation for ‘a’ to become the second animation in the list and a new animation will be created for the first item in the list.

		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;
		  }

		}

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).

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.

Declaring Keyframes

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 zero or more times. Keyframes are specified using the @keyframes at-rule, defined as follows:

		@keyframes = @keyframes <> { <> }

		<keyframes-name> = <> | <>

		<keyframe-block> = <># { <> }

		<keyframe-selector> = from | to | <>

The <> inside of ''@keyframes'' can only contain <> rules. The <> inside of <> accepts any CSS property except those defined in this specification, but does accept the 'animation-timing-function' property and interprets it specially. None of the properties interact with the cascade (so using ''!important'' on them is invalid and will cause the property to be ignored). A ''@keyframes'' block has a name given by the <> or <> in its prelude. The two syntaxes are equivalent in functionality; the name is the value of the ident or string. As normal for <>s and <>s, the names are fully case-sensitive; two names are equal only if they are codepoint-by-codepoint equal. The <> additionally excludes the ''animation-name/none'' keyword.

For example, the following two ''@keyframes'' rules have the same name, so the first will be ignored:

		@keyframes foo { /* ... */ }
		@keyframes "foo" { /* ... */ }

On the other hand, the following ''@keyframes'' rule's name is different from the previous two rules:

		@keyframes FOO { /* ... */ }

The following ''@keyframes'' rules are invalid because they use disallowed <> values:

		@keyframes initial { /* ... */ }
		@keyframes None { /* ... */ }

However, those names can be specified with a <>, so the following are both valid:

		@keyframes "initial" { /* ... */ }
		@keyframes "None" { /* ... */ }

The <> for a <> 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%''. Values less than ''0%'' or higher than ''100%'' are invalid and cause their <> to be ignored. Note: Note that the percentage unit specifier must be used on percentage values. Therefore, ''0'' is an invalid keyframe selector. 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. The <> contains 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, properties qualified with !important are invalid and ignored. If multiple ''@keyframes'' rules are defined with the same name, the last one in document order wins, and all preceding ones are ignored.

		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;
		  }

		}

The two 50% rules from above can also be combined into an equivalent single rule as illustrated below:

		@keyframes slide-right {

		  from {
		    margin-left: 0px;
		  }

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

		  to {
		    margin-left: 200px;
		  }

		}

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.

		@keyframes wobble {
		  0% {
		    left: 100px;
		  }

		  40% {
		    left: 150px;
		  }

		  60% {
		    left: 75px;
		  }

		  100% {
		    left: 100px;
		  }
		}

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''.

This specification needs to define how the value is determined from the keyframes, like the section on Application of transitions does for CSS Transitions.

Timing functions for keyframes

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

		@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;
		  }

		}

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.

A timing function specified on the ''to'' or ''100%'' keyframe is ignored. See the 'animation-timing-function' property for more information.

Declaring Animations

CSS Animations are defined by binding keyframes to an element using the 'animation-*' properties. These list-valued properties, which are all [=longhands=] of the 'animation' [=shorthand=], form a [=coordinating list property group=] with 'animation-name' as the [=coordinating list base property=] and each item in the [=coordinated value list=] defining the properties of a single animation effect. Note: This is analogous to the behavior of the 'background-*' properties, with 'background-image' analogous to 'animation-name'. See [[css-values-4#linked-properties]] for how the individual 'animation-*' property values coordinate.

The 'animation-name' property

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 none 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.

	Name: animation-name
	Value: [ none | <> ]#
	Initial: none
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: list, each item either a case-sensitive css identifier or the keyword ''animation-name/none''
	Animation type: not animatable
	Canonical order: per grammar

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

none: No keyframes are specified at all, so there will be no animation. Any other animations properties specified for this animation have no effect.
<>: The animation will use the keyframes with the name specified by the <>, if they exist. If no ''@keyframes'' rule with that name exists, there is no animation.

The 'animation-duration' property

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

	Name: animation-duration
	Value: <>#
	Initial: 0s
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: list, each item a duration
	Animation type: not animatable
	Canonical order: per grammar

<>: Specifies the length of time that an animation takes to complete one cycle. A negative <> is invalid. If the <> 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'. After the 'animation-delay' 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'' the animation will have no visible effect.

The 'animation-timing-function' property

The 'animation-timing-function' property describes how the animation will progress between each pair of keyframes. Timing functions are defined in the separate CSS Easing Functions module [[!css-easing-1]]. The input progress value used is the percentage of the time elapsed between the current keyframe and the next keyframe after incorporating the effect of the 'animation-direction' property. During the 'animation-delay', the 'animation-timing-function' is not applied. Note: This definition is necessary because otherwise a step easing function with a step position of start would produce a backwards fill equal to the top of the first step in the function. The output progress value is used as the p value when interpolating the property values between the current and next keyframe.

	Name: animation-timing-function
	Value: <>#
	Initial: ease
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: list, each item a computed <>
	Animation type: not animatable
	Canonical order: per grammar

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 'animation-iteration-count' property

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 (duration x iteration-count) is known as the active duration.

	Name: animation-iteration-count
	Value: <>#
	Initial: 1
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: list, each item either a number or the keyword ''animation-iteration-count/infinite''
	Animation type: not animatable
	Canonical order: per grammar

<single-animation-iteration-count> = infinite | <>

infinite

The animation will repeat forever.

<>

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.

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

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

The 'animation-direction' property

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.

	Name: animation-direction
	Value: <>#
	Initial: normal
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: list, each item a keyword as specified
	Animation type: not animatable
	Canonical order: per grammar

<single-animation-direction> = normal | reverse | alternate | alternate-reverse

normal: All iterations of the animation are played as specified.
reverse: All iterations of the animation are played in the reverse direction from the way they were specified.
alternate: 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.
alternate-reverse: 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.

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

The 'animation-play-state' property

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

	Name: animation-play-state
	Value: <>#
	Initial: running
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: list, each item a keyword as specified
	Animation type: not animatable
	Canonical order: per grammar

<single-animation-play-state> = running | paused

running: While this property is set to ''running'', the animation proceeds as normal.
paused: 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''.

The 'animation-delay' property

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 before it is applied.

	Name: animation-delay
	Value: <>#
	Initial: 0s
	Applies to: all elements
	Inherited: no
	Animation type: not animatable
	Percentages: N/A
	Computed value: list, each item a duration
	Canonical order: per grammar

<>: The <> 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 valid. 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 active duration. 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.

The 'animation-fill-mode' property

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.

	Name: animation-fill-mode
	Value: <>#
	Initial: none
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: list, each item a keyword as specified
	Animation type: not animatable
	Canonical order: per grammar

<single-animation-fill-mode> = none | forwards | backwards | both

none: The animation has no effect when it is applied but not executing.
forwards: 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 ''animation-fill-mode/backwards'').
backwards: 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 ''animation-direction/normal'' or ''animation-direction/alternate'') or those of the ''to'' keyframe (when 'animation-direction' is ''animation-direction/reverse'' or ''animation-direction/alternate-reverse'').
both: The effects of both ''animation-fill-mode/forwards'' and ''animation-fill-mode/backwards'' fill apply.

The 'animation' shorthand property

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.)

	Name: animation
	Value: 	<>#
	Initial: see individual properties
	Applies to: all elements
	Inherited: no
	Percentages: N/A
	Computed value: see individual properties
	Animation type: not animatable
	Canonical order: per grammar

<single-animation> = <> || <> || <> || <> || <> || <> || <> || [ none | <> ] Order is important within each animation definition: the first value in each <> that can be parsed as a <> is assigned to the 'animation-duration', and the second value in each <> that can be parsed as a <> is assigned to 'animation-delay'. Order is also important within each animation definition for distinguishing <> 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.

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'').

Animation Events

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.

The `AnimationEvent` Interface

The AnimationEvent interface provides specific contextual information associated with Animation events.

IDL Definition

		[Exposed=Window]
		interface AnimationEvent : Event {
		  constructor(CSSOMString type, optional AnimationEventInit animationEventInitDict = {});
		  readonly attribute CSSOMString animationName;
		  readonly attribute double elapsedTime;
		  readonly attribute CSSOMString pseudoElement;
		};
		dictionary AnimationEventInit : EventInit {
		  CSSOMString animationName = "";
		  double elapsedTime = 0.0;
		  CSSOMString pseudoElement = "";
		};

Attributes

animationName: The value of the 'animation-name' property of the animation that fired the event.
elapsedTime: The amount of time the animation has been running, in seconds, when this event fired, excluding any time the animation was paused. The precise calculation for of this member is defined along with each event type.
pseudoElement: 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).

AnimationEvent(type, animationEventInitDict) is an event constructor.

Types of `AnimationEvent`

The different types of animation events that can occur are:

animationstart

The {{animationstart}} 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 {{AnimationEvent/elapsedTime}} equal to the absolute value of the delay capped to the active duration of the animation, that is, min(max(-'animation-delay', 0), active duration); in this case the event will fire whether 'animation-play-state' is set to ''running'' or ''paused''.

Bubbles: Yes
Cancelable: No
Context Info: animationName, elapsedTime, pseudoElement

animationend

The {{animationend}} event occurs when the animation finishes. In this case the value of the {{AnimationEvent/elapsedTime}} member of the event is equal to the active duration.

Bubbles: Yes
Cancelable: No
Context Info: animationName, elapsedTime, pseudoElement

animationiteration

The {{animationiteration}} 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.

The {{AnimationEvent/elapsedTime}} member in this case is equal to the product of the current iteration and 'animation-duration' where the current iteration is the zero-based index of the new iteration. For example, assuming no negative 'animation-delay', after one iteration completes the current iteration would be one.

Bubbles: Yes
Cancelable: No
Context Info: animationName, elapsedTime, pseudoElement

animationcancel

The {{animationcancel}} event occurs when the animation stops running in a way that does not fire an {{animationend}} event, such as a change in the 'animation-name' that removes the animation, or the animating element or one of its ancestors becoming ''display:none''.

The {{AnimationEvent/elapsedTime}} member for this event indicates the number of seconds that had elapsed since the beginning of the animation at the moment when the animation was canceled. This excludes any time where the animation was paused. If the animation had a negative 'animation-delay', the beginning of the animation is the moment equal to the absolute value of 'animation-delay' seconds prior to when the animation was actually triggered. Alternatively, if the animation had a positive 'animation-delay' and the event is fired before the animation's delay has expired, the {{AnimationEvent/elapsedTime}} will be zero.

Bubbles: Yes
Cancelable: No
Context Info: animationName, elapsedTime, pseudoElement

Event handlers on elements, `Document` objects, and `Window` objects

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

Event handler	Event handler event type
onanimationstart	{{animationstart}}
onanimationiteration	{{animationiteration}}
onanimationend	{{animationend}}
onanimationcancel	{{animationcancel}}

DOM Interfaces

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.

The `CSSRule` Interface

The following two rule types are added to the {{CSSRule}} interface. They provide identification for the new keyframe and keyframes rules.

IDL Definition

	partial interface CSSRule {
		  const unsigned short KEYFRAMES_RULE = 7;
		  const unsigned short KEYFRAME_RULE = 8;
	};

The `CSSKeyframeRule` Interface

The {{CSSKeyframeRule}} interface represents the style rule for a single key.

IDL Definition

	[Exposed=Window]
	interface CSSKeyframeRule : CSSRule {
	  attribute CSSOMString keyText;
	  [SameObject, PutForwards=cssText] readonly attribute CSSStyleProperties style;
	};

Attributes

keyText

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 keyText is updated with an invalid keyframe selector, a SyntaxError exception must be thrown and the value of keyText must remain unchanged.

style

Must return a {{CSSStyleProperties}} object for the keyframe rule, with the following properties:

readonly flag: Unset.
declarations: The declared declarations in the rule, in specified order.
parent CSS rule: The context object (i.e. this {{CSSKeyframeRule}}).
owner node: Null.

The `CSSKeyframesRule` Interface

The {{CSSKeyframesRule}} interface represents a complete set of keyframes for a single animation.

IDL Definition

	[Exposed=Window]
	interface CSSKeyframesRule : CSSRule {
	           attribute CSSOMString name;
	  readonly attribute CSSRuleList cssRules;
	  readonly attribute unsigned long length;

	  getter CSSKeyframeRule (unsigned long index);
	  undefined        appendRule(CSSOMString rule);
	  undefined        deleteRule(CSSOMString select);
	  CSSKeyframeRule? findRule(CSSOMString select);
	};

Attributes

name: This attribute is the name of the keyframes, used by the 'animation-name' property.
cssRules: This attribute gives access to the keyframes in the list.
length: This attribute is the number of keyframes in the list.

The indexed property getter

The indexed property getter returns the {{CSSKeyframeRule}} from the list of keyframes at the indicated position. Parameters:

index of type {{unsigned long}}: The zero-based index of the rule to return.

Return Value:

{{CSSKeyframeRule}}: The found rule or {{undefined}} if there is no rule at the specific index.

No Exceptions

The `appendRule` method

The appendRule method appends the passed {{CSSKeyframeRule}} at the end of the keyframes rule. Parameters:

rule of type {{CSSOMString}}: 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.

No Return Value No Exceptions

The `deleteRule` method

The deleteRule method deletes the last declared {{CSSKeyframeRule}} matching the specified keyframe selector. If no matching rule exists, the method does nothing. Parameters:

select of type {{CSSOMString}}: 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.

No Return Value No Exceptions

The `findRule` method

The findRule returns the last declared {{CSSKeyframeRule}} matching the specified keyframe selector. If no matching rule exists, the method does nothing. Parameters:

select of type {{CSSOMString}}: The keyframe selector of the rule to be found: 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.

Return Value:

{{CSSKeyframeRule}}: The found rule.

No Exceptions

For example, given the following animation:

		@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;
		  }

		}

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

		anim.deleteRule('10%');
		var tenPercent = anim.findRule('10%');

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 tenPercent. The following:

		var red = anim.findRule('75%');

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

		var red = anim.findRule('25%,75%');

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

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

Extensions to the `GlobalEventHandlers` Interface Mixin

This specification extends the {{GlobalEventHandlers}} interface mixin from HTML to add event handler IDL attributes for animation events as defined in [[#event-handlers-on-elements-document-objects-and-window-objects]].

IDL Definition

	partial interface mixin GlobalEventHandlers {
	  attribute EventHandler onanimationstart;
	  attribute EventHandler onanimationiteration;
	  attribute EventHandler onanimationend;
	  attribute EventHandler onanimationcancel;
	};

Privacy Considerations

No privacy concerns have been reported on this specification.

Security Considerations

No security concerns have been reported on this specification.

Changes

Changes since the Working Draft of 11 October 2018

The following substantive changes were made:

Defined indexed property getter for CSSKeyframesRule
Added constructor type on AnimationEvent's definition
Added required unit for dimension in range notation
Applied range definition notation to descriptor and rule's prelude values
Applied range definition notation to property values
Associated event definitions with their EventHandler container
Better markup for productions
Corrected typo (rule to be found, not rule to be deleted)
Made value definition reference consistent with other CSS specifications
IDL aligned with Web IDL specification
Added default dictionary value to constructor
Rewrote confusing example (#4118)
Clarified handling of zero-duration animations
Use "not animatable" rather than "none"
Timing functions now called easing functions
Changed GlobalEventHandlers to be a mixin

Acknowledgements

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.

Other open issues

Need to specify how keyframes interact.

Working Group Resolutions that are pending editing

This section is informative and temporary. The editors are currently behind on editing this spec. The following working group resolutions still need to be edited in:

2014-09-09 minutes (Antibes f2f)
- Issue(7335): Detail how/when keyframe values are computed; using G.beta in dbaron's mail
- 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
- ~~Dynamic changes to animation properties/keyframes. Tab to propose resolution. (Bug 14713)~~
- 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)
- ~~Fire animation start/end events when animation-duration is zero, with 0 elapsedTime~~
- ~~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 Brian's mail)~~
- ~~If an animation with a negative animation delay is initially paused, the start event still fires~~
2012-10-29 minutes
- ~~Change the animation properties to be dynamically changeable~~
- ~~@keyframes can be dynamically changed~~
- ~~When you encounter duplicate animations names, last one wins.~~
- 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.
2012-12-12 minutes and intermediate comments ~~and 2012-12-19 minutes~~
- ~~Animations only run if they contain at least one valid keyframe rule (Bug)~~
- ~~When an element changes from display:none to display: non-none, animations start immediately (Bug)~~
- ~~An initially-paused animation is still started (fires start events etc.) (Bug)~~
- ~~Animations can be paused during their delay phase, which freezes the remaining delay to be applied after it unpauses (Bug)~~
- ~~animation-play-state has the same list behavior as the other animation properties, matching the length of animation-name (Bug)~~
2013-02-20 minutes
- ~~Øyvind's clarification accepted~~
- ~~keyframe rules cascade~~
- ~~mark pseudoElement at-risk~~
2013-05-30 minutes
- ~~expectations on animations in non-interactive media~~
2014-01-27 minutes
- ~~remove text about waiting for document load~~

Introduction

Value Definitions

CSS Animations Model

Declaring Keyframes

Timing functions for keyframes

Declaring Animations

The 'animation-name' property

The 'animation-duration' property

The 'animation-timing-function' property

The 'animation-iteration-count' property

The 'animation-direction' property

The 'animation-play-state' property

The 'animation-delay' property

The 'animation-fill-mode' property

The 'animation' shorthand property

Animation Events

The AnimationEvent Interface

IDL Definition

Attributes

Types of AnimationEvent

Event handlers on elements, Document objects, and Window objects

DOM Interfaces

The CSSRule Interface

IDL Definition

The CSSKeyframeRule Interface

IDL Definition

Attributes

The CSSKeyframesRule Interface

IDL Definition

Attributes

The indexed property getter

The appendRule method

The deleteRule method

The findRule method

Extensions to the GlobalEventHandlers Interface Mixin

IDL Definition

Privacy Considerations

Security Considerations

Changes

Changes since the Working Draft of 11 October 2018

Acknowledgements

Other open issues

Working Group Resolutions that are pending editing

The `AnimationEvent` Interface

Types of `AnimationEvent`

Event handlers on elements, `Document` objects, and `Window` objects

The `CSSRule` Interface

The `CSSKeyframeRule` Interface

The `CSSKeyframesRule` Interface

The `appendRule` method

The `deleteRule` method

The `findRule` method

Extensions to the `GlobalEventHandlers` Interface Mixin