CSS Animations

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.

Status of this document

Table of contents

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.

Animations

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.

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.

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 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 the element has a value for 'animation-name' that references a valid keyframes rule. 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;
      }

    }

    

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.

The keyframe declaration 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.

NOTE: 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.

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 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.

Example(s):

  @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 '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_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;
    }

  }

  

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

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 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. In the case where 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 transitions. The lists are matched up from the first value: excess values at the end are not used. If one of the other properties doesn't have enough comma-separated values to match the number of values of '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'.

The 'animation-duration' Property

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

By default the value is '0s', meaning that the animation cycle is immediate. A negative value for 'animation-duration' is treated as '0s'. In this case '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 fill forwards will retain the value specified at the 100% keyframe, even if the animation was instantaneous. Also, animation events are still fired.

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.

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.

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 of 'animation-iteration-count' are invalid. This property is often used with an 'animation-direction' value of alternate, which will cause the animation to play in reverse on alternate cycles.

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.

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.

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.

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.

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 value of 'animation-fill-mode' can override this behavior.

If the value for 'animation-fill-mode' is 'backwards', then the animation will apply the property values defined in its 0% or 'from' keyframe as soon as the animation is applied, during the period defined by 'animation-delay'.

If the value for 'animation-fill-mode' is 'forwards', then the animation will apply the property values defined in its last executing keyframe after the final iteration of the animation, until the animation style is removed. The last executing keyframe is the 'to' or '100%' keyframe, unless the animation has 'animation-direction' set to 'alternate' and both a finite and even iteration count, in which case it is the 'from' or '0%' keyframe.

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 properties in both directions.

The 'animation' Shorthand Property

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

Note that order is important in this property. The first value that can be parsed as a time is assigned to the animation-duration. The second value that can be parsed as a time is assigned to animation-delay.

An alternative proposal is to accept the font shorthand approach of using a "/" character between the values of the same type. e.g. 2s/4s would mean a duration of 2 seconds and a delay of 4 seconds.

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. 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).

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. 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

animationend

The 'animationend' event occurs when the animation finishes.

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

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

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               appendRule(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

appendRule

The appendRule method appends the passed CSSKeyframeRule into the list at the passed key.

Parameters

rule of type DOMString

The rule to be appended, expressed in the same syntax as one entry in the @keyframes 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

Acknowledgments

Thanks especially to the feedback from Tab Atkins, Estelle Weyl, and all the rest of the www-style community.

References

Normative references

Other references

Property index

Index