CSS Animations Level 1

Status: ED
Shortname: css-animations
Level: 1
Group: csswg
TR: http://www.w3.org/TR/css3-animations/
ED: http://dev.w3.org/csswg/css3-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, dbaron@dbaron.org
Editor: Sylvain Galineau, Adobe, galineau@adobe.com
Former Editor: David Hyatt, Apple Inc.
Former Editor: Chris Marrin, Apple Inc.

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. 

Ignored Terms: domstring, float, animationeventinit, eventinit, eventtarget, document

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.

Values

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 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.

Animations

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 whose name 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.

The diagram above shows how property values are computed. The intrinsic style is shown at the top of the diagram. The computed value is derived from intrinsic style at the times when an animation is not running and also when an animation is delayed (see below for specification of animation delay). During an animation, the computed style is derived from the animated value. The start time of an animation is the latter of two moments: the time at which the style is resolved that specifies the animation, or the time the document's load event is fired. Therefore, an animation specified in the document style sheet will begin at the document load. An animation specified on an element by modifying the style after the document has loaded will start when the 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). An animation applies to an element if its name appears as one of the identifiers in the computed value of the 'animation-name' property. Once an animation has started it continues until it ends or the 'animation-name' is removed. The values used for the keyframes and animation properties are snapshotted at the time the animation starts. Changing them during the execution of the animation has no effect. 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 {
		  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 ''none'' will terminate any running animation applied to the element and its descendants. If an element has a display of ''none'', updating display to a value other than ''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 ''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]]).

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 one or more times. Keyframes are specified using a specialized CSS at-rule. A @keyframes 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%’. 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. 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. Properties that are unable to be animated 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. Issue: Need to describe what happens if a property is not present in all keyframes. 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. @keyframes rules do not cascade; therefore, an animation will never derive keyframes from more than one @keyframes rule. Note: Note that since empty @keyframes rule are valid, they may hide the keyframes of those preceding animation definitions with a matching name. To determine the set of keyframes, all of the values in the selectors are sorted in increasing order by time. If there are any duplicates, then the last keyframe specified inside the @keyframes rule will be used to provide the keyframe information for that time. There is no cascading within a @keyframes rule if multiple keyframes specify the same keyframe selector values. 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.

The following is the grammar for the keyframes rule:

		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;}

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.

The 'animation-name' property

	Name: animation-name
	Value: <> [',' <>]*
	Initial: none
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: none
	Animatable: no
	Percentages: N/A
	Media: visual
	Computed value: As specified
	Canonical order: per grammar

<single-animation-name> = none | <>

The 'animation-duration' property

The ‘animation-duration’ property defines the length of time that an animation takes to complete one cycle.

	Name: animation-duration
	Value: <> [, <>]*
	Initial: 0s
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: visual
	Computed value: As specified
	Canonical order: per grammar

The initial value is 0s, meaning that the animation takes no time. When the duration is 0s 'animation-fill-mode' still applies, so an animation that fills backwards will show the value of the 0% keyframe during any delay period, and an animation that fills forwards will retain the value specified at the 100% keyframe, even if the animation was instantaneous. Also, animation events are still fired. A negative 'animation-duration' value renders the declaration invalid.

The 'animation-timing-function' property

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.

	Name: animation-timing-function
	Value: <> [ ‘,’ <> ]* 
	Initial: ease
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: visual
	Computed value: As specified
	Canonical order: per grammar

All the valid values of <single-timing-function> are defined by the 'transition-timing-function' property [[!CSS3-TRANSITIONS]]. For a keyframed animation, the 'animation-timing-function' applies between keyframes, not over the entire animation. For example, in the case of an ease-in-out timing function, an animation will ease in at the start of the keyframe and ease out at the end of the keyframe. An 'animation-timing-function' defined within a keyframe block applies to that keyframe, otherwise the timing function specified for the animation is used. In addition, only the first value of the property applies when it is used in a keyframe block.

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. A value of ''infinite'' will cause the animation to repeat forever. Non-integer numbers will cause the animation to end part-way through a cycle. Negative values of 'animation-iteration-count' are invalid. This property is often used in conjunction an 'animation-direction' value of ‘alternate’, which will cause the animation to play in reverse on alternate cycles.

	Name: animation-iteration-count
	Value: <> [ ‘,’ <> ]* 
	Initial: 1
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: visual
	Computed value: As specified
	Canonical order: per grammar

<single-animation-iteration-count> = 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, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: visual
	Computed value: As specified
	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: Note that 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. A running animation can be paused by setting this property to ''paused''. To continue running a paused animation this property can be set to ''running''. A paused animation will continue to display the current value of the animation in a static state, as if the time of the animation is constant. When a paused animation is resumed, it restarts from the current value, not necessarily from the beginning of the animation.

	Name: animation-play-state
	Value: <> [ , <> ]*  
	Initial: running
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: visual
	Computed value: As specified
	Canonical order: per grammar

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

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. An 'animation-delay' value of ‘0s’ means the animation will execute as soon as it is applied. Otherwise, the value specifies an offset from the moment the animation is applied, and the animation will delay execution by that offset. If the value for 'animation-delay' is a negative time offset then the animation will execute the moment it is applied, but will appear to have begun execution at the specified offset. That is, the animation will appear to begin part-way through its play cycle. In the case where an animation has implied starting values and a negative 'animation-delay', the starting values are taken from the moment the animation is applied.

	Name: animation-delay
	Value: <> [ , <>]* 
	Initial: 0s
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: visual
	Computed value: As specified
	Canonical order: per grammar

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' property). The 'animation-fill-mode' property can override this behavior. If the value for 'animation-fill-mode' is ''backwards'', then the animation will apply the property values defined in the keyframe that will start the first iteration of the animation, during the period defined by 'animation-delay'. 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''). If the value for 'animation-fill-mode' is ''forwards'', then 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''). If the value for 'animation-fill-mode' is ''both'', then the animation will follow the rules for both ''forwards'' and ''backwards''. That is, it will extend the animation in both directions.

	Name: animation-fill-mode
	Value: <> [ , <> ]*  
	Initial: none
	Applies to: all elements, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: visual
	Computed value: As specified
	Canonical order: per grammar

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

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, ::before and ::after pseudo-elements
	Inherited: no
	Animatable: no
	Percentages: N/A
	Media: visual
	Computed value: As specified
	Canonical order: per grammar

<single-animation> = <> || <> || <> || <> || <> || <> || <> || <> Note that 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'. Note that 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' 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.

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 both a valid keyframe rule and a non-zero duration are 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

		[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 = "";
		};

Attributes

animationName, of type DOMString, readonly: The value of the 'animation-name' property of the animation that fired the event.
elapsedTime, of type float, readonly: 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).
pseudoElement, of type DOMString, readonly: 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 elapsedTime equal to the absolute value of the delay.

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

animationend

The ''animationend'' event occurs when the animation finishes.

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.

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