Title: CSS Animations Level 2
Status: ED
Work Status: Exploring
Shortname: css-animations-2
Level: 2
Group: csswg
ED: https://drafts.csswg.org/css-animations-2/
Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Editor: Brian Birtles, Mozilla https://www.mozilla.org/, bbirtles@mozilla.com, w3cid 43194
!Issues List: In Bugzilla

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 Vars: auto-rewind

urlPrefix: https://w3c.github.io/web-animations/; type: interface; spec: web-animations
    text: Animation
    text: KeyframeEffectReadOnly
urlPrefix: https://w3c.github.io/web-animations/; type: method; for: Animation; spec: web-animations
    text: play()
    text: pause()
urlPrefix: https://w3c.github.io/web-animations/; type: method; for: KeyframeEffectReadOnly; spec: web-animations
    text: getFrames()
urlPrefix: https://w3c.github.io/web-animations/; type: dfn; spec: web-animations
    text: active duration
    text: active phase
    text: active time
    text: after phase
    text: animation
    text: animation class
    text: animation effect
    text: animation playback rate
    text: before phase
    text: composite operation
    text: current iteration
    text: current time
    text: fill mode
    text: global animation list
    text: idle play state
    text: idle phase
    text: iteration duration
    text: iteration start
    text: pause an animation
    text: pending play state
    text: play an animation
    text: sampling
    text: start delay
    text: target element
    text: target effect
    text: target effect end
    text: unresolved

Delta specification

This is a delta specification, meaning that it currently contains only the differences from CSS Animations Level 1 [[!CSS3-ANIMATIONS]]. Once the Level 1 specification is closer to complete, it will be merged with the additions here into a complete level 2 specification.

# Animations # {#animations} ## Owning element ## {#owning-element-section} The owning element of an animation refers to the element or pseudo-element to which the 'animation-name' property was applied that generated the animation. If an animation was generated directly by script (e.g. using the {{CSSAnimation}} constructor) then it has no owning element. If an animation generated using the markup defined in this specification is later disassociated from that markup by an update to the computed value of the 'animation-name' property on the owning element, the animation is disassociated from its owning element (that is, it has no owning element from that point forwards).
In the example below, animation's initial owning element is elem. animation is disassociated from element through an update to the computed value of elem's 'animation-name' property. 
elem.style.animation = 'spin 1s';
let animation = elem.getAnimations()[0]; // animation's owning element is elem
elem.style.animation = ''; // animation no longer has an owning element
Note that although the owning element is often equal to the target element of an animation's target effect, this is not always the case. The following example demonstrates some of the situations where these two elements may differ. 
elem.style.animation = 'move 1s';
let animation = elem.getAnimations()[0];
// animation.effect.target == elem == animation's owning element

let mutableEffect = animation.effect.clone();
animation.effect = mutableEffect;
animation.effect.target = elem2;
// animation.effect.target == elem2 != animation's owning element

animation.effect = null;
// animation.effect.target is undefined != animation's owning element
## Animation composite order ## {#animation-composite-order} Animations generated from the markup or interfaces (e.g. the {{CSSAnimation}} constructor) defined in this specification have an animation class of ‘CSS Animation’. CSS Animations with an owning element have a later composite order than CSS Transitions but an earlier composite order than animations without a specific animation class. Within the set of CSS Animations with an owning element, two animations A and B are sorted in composite order (first to last) as follows: 1. If the owning element of A and B differs, sort A and B by tree order of their corresponding owning elements. With regard to pseudo-elements, the sort order is as follows: * element * ::before * ::after * element children 1. Otherwise, sort A and B based on their position in the computed value of the 'animation-name' property of the (common) owning element. The composite order of CSS Animations without an owning element is based on their position in the global animation list. Issue: This differs from the behavior defined for transitions. We should probably sort transitions first, then animation, then use the global animation list. The reason being that when developer tools etc. hang on to orphaned animations and transitions in order to replay them, they should maintain roughly the same composite order. CSS Animations generated using the markup defined in this specification are not added to the global animation list when they are created. Instead, these animations are appended to the global animation list at the first moment when they transition out of the idle play state after being disassociated from their owning element. CSS Animations that have been disassociated from their owning element but are still idle do not have a defined composite order. Note, this behavior relies on the fact that disassociating an animation from its owning element always causes it to enter (or remain) in the idle play state. CSS Animations created using the {{CSSAnimation}} constructor are appended to the global animation list at the moment they are constructed. # Keyframes # {#keyframes} ## The 'animation-play-state' property ## {#animation-play-state} ### Interaction between 'animation-play-state' and the Web Animations API ### {#interaction-between-animation-play-state-and-web-animations-API} Both this specification and the Web Animations specification [[WEB-ANIMATIONS]] define mechanisms for pause control, specifically the 'animation-play-state' property, and the {{Animation/play()}} and {{Animation/pause()}} methods respectively.
The interaction of these methods can be summarized as follows: * Calling {{Animation/pause()}} is sticky. The animation will remain paused until there is a subsequent call to {{Animation/play()}} after which point the animation will begin tracking changes to the computed value of the 'animation-play-state' property on the owning element from the next moment it becomes newly ''paused''. * If the computed 'animation-play-state' on the owning element is ''paused'', calling {{Animation/play()}} will temporarily override the 'animation-play-state' until it next becomes ''running''.
With regards to the pausing, an animation can be considered to be in one of five mutually-exclusive states: A state transition chart follows:
Initial state
Event A B C D E
Resulting state {{Animation/play()}} A B A B B
{{Animation/pause()}} C D C D D
'animation-play-state' → ''running'' A A C C A
'animation-play-state' → ''paused'' E B D D E
If any change causes an animation to change from one of the running states (A, B) to one of the paused states (C, D, E), the user agent must run the pause an animation procedure on the animation. Likewise, for any change from one of the the paused states to one of the running states, the user agent must run the play an animation procedure on the animation. If the change is due to a change in the computed value of 'animation-play-state' (i.e. the transition E → A) the auto-rewind flag for that procedure must be set to false; otherwise it must be set to true. ## The 'animation-composition' property ## {#animation-composition} The 'animation-composition' property defines the composite operation used when multiple animations affect the same property simultaneously. 
Name: animation-composition
Value: <>#
Initial: replace
Applies to: all elements
Inherited: none
Percentages: N/A
Computed value: list, each item a keyword as specified
Animation type: not animatable
Canonical order: per grammar
<single-animation-composition> = replace | add | accumulate The values of 'animation-composition' have the meaning defined for the corresponding values of the composite operation defined in Web Animations [[!WEB-ANIMATIONS]]. When specified in a keyframe, 'animation-composition' defines the composite operation to use for each property specified in that keyframe until the next keyframe specifying each property.
For example, the following stylesheet defines two different animations targetting the 'scale' property. 
    @keyframes heartbeat {
      from {
        scale: 1;
        animation-timing-function: ease-out;
      }
      30% {
        scale: 1.3;
      }
    }
    .heartbeat {
      animation: heartbeat 0.3s 2s infinite;
    }

    @keyframes throb {
      50% {
        scale: 1.8;
      }
    }
    .icon:mouseover {
      animation: throb 0.4s add;
    }
If these two animations are applied to the same element, normally only one animation would apply, but by specifying ''add'' as the 'animation-composition' on the second animation, the result of the two animations will be combined. Since CSS Transitions [[CSS3-TRANSITIONS]] have a lower composite order, it is possible to use 'animation-composition' to combine CSS Animations with underlying transitions as in the following example. 
    .icon {
      filter: blur(20px);
      transition: filter 0.5s;
    }
    .icon:hover {
      filter: blur(0px);
      animation: brightness-pulse 3s infinite add;
    }

    @keyframes pulse {
      0% {
        scale: 1.1;
        filter: brightness(130%);
      }
      10% {
        scale: 1;
        filter: brightness(100%);
      }
    }
Issue: Create pictures of these examples and verify they make sense.
# Animation Events # {#events} ## Event dispatch ## {#event-dispatch} Note, this is a more general description of event dispatch than that of CSS Animations Level 1 [[CSS3-ANIMATIONS]] since it must account for the possibility of animations being seeked or reversed using the Web Animations API [[WEB-ANIMATIONS]]. For the purpose of determining which events to dispatch, the [[web-animations#animation-effect-phases-and-states|phases]] defined in the Web Animations model are used. These definitions apply to an animation effect, however, for the purpose of dispatching events, we consider a CSS Animation to have the same phase as its target effect. For example, a CSS Animation is in the before phase if its target effect is in the before phase. A CSS Animation that does not have a target effect is considered to be in the idle phase if its current time is unresolved, in the before phase if its current time is less than zero, and in the after phase otherwise. Similarly, subsequent references to the start delay, active duration, current iteration, iteration start, and iteration duration of a CSS animation should be understood to refer to the corresponding properties of the animation's target effect. For calculating the {{AnimationEvent/elapsedTime}} of each event, the following definitions are used: * interval start = max(min(-start delay, active duration), 0) * interval end = max(min(target effect end - start delay, active duration), 0) Each time an animation is sampled and is not pending, the events to dispatch are determined by comparing the animation's phase before and after the sample as follows:
Change Events dispatched Elapsed time (ms)
idle or before → active {{animationstart}} interval start
idle or before → after ٭ {{animationstart}} interval start
{{animationend}} interval end
active → before {{animationend}} interval start
active → active and the current iteration of the animation's target effect has changed since the previous sample {{animationiteration}} (See below) †
active → after {{animationend}} interval end
after → active {{animationstart}} interval end
after → before ٭ {{animationstart}} interval end
{{animationend}} interval start
not idle and not after→ idle {{animationcancel}} The active time of the animation at the moment it was cancelled calculated using a fill mode of both.

٭ Where multiple events are listed for a state change, all events are dispatched in the order listed and in immediate succession.

† The elapsed time for an {{animationiteration}} event is defined as follows:

1. Let previous current iteration be the current iteration from the previous sample. 1. If previous current iteration is greater than current iteration, let iteration boundary be current iteration + 1, otherwise let it be current iteration. 1. The elapsed time is the result of evaluating (iteration boundary - iteration start) × iteration duration). Since the elapsed time defined in the table and procedure above is expressed in milliseconds, it must be divided by 1,000 to produce a value in seconds before being assigned to the {{AnimationEvent/elapsedTime}} member of the {{AnimationEvent}}. # DOM Interfaces # {#interface-dom} ## The CSSAnimation interface ## {#the-CSSAnimation-interface} 
[Exposed=Window]
interface CSSAnimation : Animation {
  readonly attribute CSSOMString animationName;
};
: animationName :: The key used to find matching keyframes rules that define target effect at the point when the animation was created. This is the value of the 'animation-name' property that caused this object to be generated or, if this object was generated using the programming interface, the animationName argument that was passed to the {{CSSAnimation}} constructor.
We need to define a constructor for {{CSSAnimation}}. Perhaps something like the following: 
[Constructor (Animatable? target,
              CSSOMString animationName,
              optional (unrestricted double or KeyframeEffectOptions) options,
              optional CSSOMString defaultEasing = "ease"),
 Constructor (Animatable? target,
              CSSOMString animationName,
              (unrestricted double or KeyframeEffectOptions) options,
              CSSOMString defaultEasing,
              AnimationTimeline? timeline)]
partial interface CSSAnimation { };
The difficulty is with liveness. The most useful and least magical (but most complicated) approach is to define a subclass of {{KeyframeEffectReadOnly}} that has the special behavior of tracking changes to all @keyframes rules that match the supplied name and automatically updating the set of keyframes returned by {{KeyframeEffectReadOnly/getFrames()}} after filling-in the default easing. Something like, 
[Exposed=Window,
 Constructor (CSSOMString keyframesName, CSSOMString defaultEasing)]
interface CSSKeyframeEffectReadOnly : KeyframeEffectReadOnly {
  readonly attribute CSSOMString keyframesName;
  readonly attribute CSSOMString defaultEasing;
};
## Requirements on pending style changes ## {#requirements-on-pending-style-changes} Various operations may affect the computed values of properties on elements. User agents may, as an optimization, defer recomputing these values until it becomes necessary. However, all operations included in programming interface defined in this specification, as well as those operations defined in Web Animations [[!WEB-ANIMATIONS]] that may return objects defined by this specification, must produce a result consistent with having fully processed any such pending changes to computed values.
As an example, in the following code fragment, when the specified style of elem is initially updated, a user agent may defer recalculating the computed value of the 'animation' property. However, the {{Animatable/getAnimations()}} method called on elem is specified by Web Animations and can return {{CSSAnimation}} objects as defined in this specification. Hence, as result of the requirements in this section, the user agent must calculate the updated value of elem's 'animation' property and create the requested {{CSSAnimation}} object before returning its result. 
elem.style.animation = 'fadeOut 1s';
elem.getAnimations()[0].pause();

Privacy and Security Considerations

This specification introduces no new privacy or security considerations.