CSS Animation

21 April 2008

Authors:
Dave Hyatt (hyatt@apple.com), Apple
Dean Jackson (dino@apple.com), Apple
Chris Marrin (cmarrin@apple.com), Apple

1 Introduction

This document introduces new CSS features to enable animation.

Animation is currently possible in languages like SVG, but no system exists for the native animation of CSS styles. This proposal introduces defined animations, which specify the values that CSS properties will take over a given time interval.

This specification is an extension to CSS Transitions.

2 Keyframes

In a simple transition, since both the starting value and ending value are known, a single timing function and duration determine the intermediate values of the animating property. If finer control of the intermediate values is required, keyframes can be used.

Keyframes are specified using a specialized CSS at-rule. A @keyframes rule consists of the keyword "@keyframes", followed by the name of the target transition (using a 'transition-name') or animation (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 or transition 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%.

The keyframe declaration for a keyframe rule consists of properties and values. Properties that are not transitioning or animating are ignored in these rules, with the exception of 'transition-timing-function' and animation-timing-function'.

The @keyframes rule that is used by a transition or animation will be the last one encountered in sorted rules order that matches the name of the transition. @keyframes rules do not cascade; therefore a transition or animation will never derive keyframes from more than one @keyframes rule.

To determine the set of keyframes, all of the values in 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.

Example(s):

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

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

Animations states specified by keyframes

The following is the grammar for the keyframes rule.

      keyframes-rule: '@keyframes' [ IDENT | STRING ] '{' keyframes-blocks '}';

      keyframes-blocks: [ keyframe-selectors block ]* ;

      keyframe-selectors: [ 'from' | 'to' | PERCENTAGE ] [ ',' [ 'from' | 'to' | PERCENTAGE ] ]*;
    

2.1 Timing functions for keyframes

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

Example(s):

@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 transition or animation named "bounce". Between the first and second keyframe (ie. between 0 and 25%) an "ease-out" timing function is used. Between the second and third keyframe (ie. 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 units up the page.

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

3 Animations

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 above.

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.

Example(s):

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).

3.1 Animation behavior

CSS Animations affect computed property values. During the execution of an animation, the computed value for a property is controlled by the animation. This overrides the value specified in the normal styling system.

In the case of multiple animations specifying behavior for the same property, the animation defined last will override the previously defined animations.

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.

Computation of animated property values

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 stylesheet 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).

3.2 The 'animation-name' Property

The 'animation-name' property defines a name for the animation. The 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 inherited animations.

Name: animation-name
Value: <string> [, <string>]*
Initial: ""
Applies to: block-level and inline-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: Same as specified value.

3.3 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: <time> [, <time>]*
Initial: 0
Applies to: block-level and inline-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: Same as specified value.

By default the value is '0', meaning that the animation cycle is immediate (i.e. there will be no animation). A negative value for animation-duration is treated as '0'.

3.4 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 for a complete description of timing function calculation.

Name: animation-timing-function
Value: ease | linear | ease-in | ease-out | ease-in-out | cubic-bezier(<number>, <number>, <number>, <number>) [, ease | linear | ease-in | ease-out | ease-in-out | cubic-bezier(<number>, <number>, <number>, <number>)]*
Initial: ease
Applies to: block-level and inline-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: Same as specified value.

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. A 'animation-timing-function' defined within a keyframe block applies to that keyframe, otherwise the timing function specified for the animation is used.

3.5 The 'animation-iteration-count' Property

The 'animation-iteration-count' property defines the number of times an animation cycle is played. The default value is one, 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 for 'animation-iteration-count' are treated as zero. This property is often used with an 'animation-direction' value of 'alternate', which will cause the animation to play in reverse on alternate cycles.

Name: animation-iteration-count
Value: infinite | <number> [, infinite | <number>]*
Initial: 1
Applies to: block-level and inline-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: Same as specified value.

3.6 The 'animation-direction' Property

The 'animation-direction' property defines whether or not the animation should play in reverse on alternate cycles. If 'alternate' is specified, 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. 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: normal | alternate [, normal | alternate]*
Initial: normal
Applies to: block-level and inline-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: Same as specified value.

3.7 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: running | paused [, running | paused]*
Initial: running
Applies to: block-level and inline-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: Same as specified value.

3.8 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 '0' 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: <time> [, <time>]*
Initial: 0
Applies to: block-level and inline-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: Same as specified value.

3.9 The 'animation' Shorthand Property

The 'animation' shorthand property combines six of the animation properties into a single property.

Name: animation
Value: [<animation-name> || <animation-duration> || <animation-timing-function> || <animation-delay> || <animation-iteration-count> || <animation-direction>] [, [<animation-name> || <animation-duration> || <animation-timing-function> || <animation-delay> || <animation-iteration-count> || <animation-direction>] ]*
Initial: see individual properties
Applies to: block-level and inline-level elements
Inherited: no
Percentages: N/A
Media: visual
Computed value: Same as specified value.

4 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.

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.

Interface AnimationEvent

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


IDL Definition
interface AnimationEvent : Event {
  readonly attribute DOMString          animationName;
  readonly attribute float              elapsedTime;
  void               initAnimationEvent(in DOMString typeArg, 
                                        in boolean canBubbleArg, 
                                        in boolean cancelableArg, 
                                        in DOMString animationNameArg,
                                        in float elapsedTimeArg);
};
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. Note that this value is not affected by the value of animation-delay. For an "animationstart" event, the elapsedTime is always zero.
Methods
initAnimationEvent
The initAnimationEvent method is used to initialize the value of an AnimationEvent created through the DocumentEvent interface. This method may only be called before the AnimationEvent has been dispatched via the dispatchEvent method, though it may be called multiple times during that phase if necessary. If called multiple times, the final invocation takes precedence.
Parameters
typeArg of type DOMString
Specifies the event type.
canBubbleArg of type boolean
Specifies whether or not the event can bubble.
cancelableArg of type boolean
Specifies whether or not the event's default action can be prevented.
animationNameArg of type DOMString
Specifies the Event's animation name.
elapsedTimeArg of type float
Specifies the amount of time, in seconds, the animation has been running at the time of initialization.
No Return Value
No Exceptions

The different types of Animation events that can occur are:

animationstart
The animationstart event occurs at the start of the animation
animationend
The animationend event occurs when the animation finishes.
animationiteration
The animationiteration event occurs at the end of each iteration of an animation for which animation-iteration-count is greater than one. This event does not occur for animations with an iteration count of one.

5 DOM Interfaces

CSS animation is exposed to the CSSOM through a pair of new interfaces describing the keyframes.

Interface CSSRule

The following 2 rule types are added to the CSSRule interface. They provide identification for the new keyframe and keyframes rules.

IDL Definition
  interface CSSRule {
    ...
    const unsigned short KEYFRAMES_RULE = 7;
    const unsigned short KEYFRAME_RULE = 8;
    ...
  };
Interface CSSKeyframeRule

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


IDL Definition
  interface CSSKeyframeRule : CSSRule {
      attribute DOMString keyText;
      readonly attribute CSSStyleDeclaration style;
  };
  

Attributes
keyText of type DOMString
This attribute represents the key as the string representation of a floating point number between 0 and 1. If the value in the CSS style is from this value will be 0, and if the value in the CSS style is to this value will be 1.
style of type CSSStyleDeclaration
This attribute represents the style associated with this keyframe.
No Methods
Interface CSSKeyframesRule

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


IDL Definition
  interface CSSKeyframesRule : CSSRule {
      attribute          DOMString   name;
      readonly attribute CSSRuleList cssRules;

      void               insertRule(in DOMString rule);
      void               deleteRule(in DOMString key);
      CSSKeyframeRule    findRule(in DOMString key);
  };
  

Attributes
name of type DOMString
This attribute is the name of the keyframes, used by the animation-name property.
cssRules of type CSSRuleList
This attribute gives access to the keyframes in the list.
Methods
insertRule
The insertRule method inserts the passed CSSKeyframeRule into the list at the passed key.
Parameters
rule of type DOMString
The rule to be inserted, expressed in the same syntax as one entry in the @keyframes rule. The key is included in the rule string, which described the point at which the rule should be inserted. If a rule with the same key already exists in the list, it is replaced with this rule.
No Return Value
No Exceptions
deleteRule
The deleteRule method deletes the CSSKeyframeRule with the passed key. If a rule with this key does not exist, the method does nothing.
Parameters
key of type DOMString
The key which describes the rule to be deleted. The key must resolve to a number between 0 and 1, or the rule is ignored.
No Return Value
No Exceptions
findRule
The findRule method returns the rule with a key matching the passed key. If no such rule exists, a null value is returned.
Parameters
key of type DOMString
The key which described the rule to find. The key must resolve to a number between 0 and 1, or the rule is ignored.
Return Value
CSSKeyframeRule
The found rule.
No Exceptions