CSS Animations Level 2

1. 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.

2. Animations

2.1. Owning element

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).

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

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

2.2. Animation composite order

Animations generated from the markup or interfaces (e.g. the CSSAnimation constructor) defined in this specification have an animation type 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 type.

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

2. 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.

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.

3. Keyframes

3.1. The animation-play-state property

3.1.1. Interaction between animation-play-state and the 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 play() and pause() methods respectively.

With regards to the pausing, an animation can be considered to be in one of five mutually-exclusive states:

• Running
• Running and temporarily overriding animation-play-state: paused
• Paused and sticky overriding animation-play-state: running
• Paused and sticky overriding animation-play-state: paused
• Paused by animation-play-state

A state transition chart follows:

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.

3.2. The animation-composition property

The animation-composition property defines the composite operation used when multiple animations affect the same property simultaneously.

<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.

@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;
}

.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%);
  }
}

4. Animation Events

4.1. Types of AnimationEvent

The additional types of animation events that can occur are:

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.

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

4.2. 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 using the Web Animations API [WEB-ANIMATIONS].

For the purpose of determining which events to dispatch, an animation can be considered to be in one of four mutually-exclusive event states determined using the following procedure:

1. If the animation is idle or has no target effect it is idle.

2. Otherwise, if the animation has a current time less than the start delay of its target effect, or, if the animation’s playback rate is less than zero and it has a current time less than or equal to the start delay of its target effect, it is left-active.

3. Otherwise, if the animation has a current time greater than its target effect end, or, if the animation’s playback rate is greater than or equal to zero and it has a current time greater than or equal to its target effect end, it is right-active.

4. Otherwise, it is active.

Each time the animation is sampled, the events to dispatch are determined by comparing the event state before and after the sample as follows:

not active → active

animationstart

left-active → right-active

right-active → left-active

animationstart, animationend

active → left-active

active → right-active

animationend

active → active

animationiteration if there has been a change to the current iteration of the animation’s target effect.

not idle → idle

animationcancel

Define the value of elapsedTime for each case.

5. DOM Interfaces

5.1. The CSSAnimation interface

interface CSSAnimation : Animation {
  readonly attribute DOMString animationName;
};

animationName, of type DOMString, readonly

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.

[Constructor (Animatable? target,
              DOMString animationName,
              optional (unrestricted double or KeyframeEffectOptions) options,
              optional DOMString defaultEasing = "ease"),
 Constructor (Animatable? target,
              DOMString animationName,
              (unrestricted double or KeyframeEffectOptions) options,
              DOMString defaultEasing,
              AnimationTimeline? timeline)]
partial interface CSSAnimation { };

[Constructor (DOMString keyframesName, DOMString defaultEasing)]
interface CSSKeyframeEffectReadOnly : KeyframeEffectReadOnly {
  readonly attribute DOMString keyframesName;
  readonly attribute DOMString defaultEasing;
};

5.2. 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.

elem.style.animation = 'fadeOut 1s';
elem.getAnimations()[0].pause();