Web Animations CSS Integration

For CSS Transitions, mappings into the Web Animations model are established pairwise for a specific CSS property, the transitioning property, on a specific element, the transitioning element.

When a CSS Transition is started for the transitioning property of a transitioning element, an Animation is constructed and associated with a Player on the document timeline, configured with a custom player priority which ensures that the Player sorts before Players created through the Web Animations API and those generated for CSS Animations. The Animation is configured as follows:

• The target element is set to the transitioning element.
• The start delay is set to the current used value of the transition-delay property.
• The iteration duration is set to the current used value of the transition-duration property.
• The timing function is set to the current used value of the transition-timing-function property.
• The animation effect is set to a keyframe animation effect with two keyframes defined as follows:
  • The first keyframe references the transitioning property with a value set to the start value of the transition.
  • The second and final keyframe references the transitioning property with a value set to the end value of the transition.

Note, changes to the values of the transition-delay, transition-duration, and transition-timing-function have no effect on a transition once it has started.

When a transition affecting the transitioning property of a transitioning element is removed or interrupted the cancel method of the corresponding Player is called.

When a transition affecting the transitioning property of a transitioning element is reversed the cancel method of the corresponding Player is called and a new Player is generated as if the transition had just begun.

2.1. CSS Transitions events

The transitionend event is triggered to fire each time the corresponding Animation enters the after phase during a sample.

//FIXME: figure out the appropriate thing to do here and remove this!

interface String { };

[MapClass(String, AnimationEffect)]
interface CSSKeyframesMap {
};

partial interface Document {
    readonly attribute CSSKeyframesMap keyframes;
};

3.1.1. The CSSKeyframesMap interface

The CSSKeyframesMap interface defines a map type that can be used to store sequences of Keyframes indexed by Strings.

3.1.2. Document.keyframes

The keyframes attribute on the Document interface references a map of type CSSKeyframesMap.

Note: The CSSKeyframesMap enables animations created using the Web Animations API to re-use the keyframe rules declared by CSS, and for animations declared by CSS to act on AnimationEffects created through the API.

Note: The processing defined in this section does not alter the way in which keyframes rules are selected when CSS animations are started, unless a name is explicitly overridden by setting its value in the map.

When a new @keyframes rule name is encountered in a style sheet, the action taken depends upon the current value stored against the name in the keyframes map.

If the current value has never been set, or has only been set due to style changes:

The keyframes defined by the @keyframes rule are converted into a KeyframeEffect (see §3.1.3 Converting a CSS Keyframes rule into a KeyframeEffect), then stored against the name in the keyframes map.

Otherwise:

The rule is ignored.

Should the KeyframeEffects generated by @keyframes rules be read-only?

When the used @keyframes rule associated with a given name changes due to a change in a style sheet, the action taken depends upon the current value stored against the name in the keyframes map.

If the current value has only been set due to style changes:

The keyframes defined by the new rule are converted into a KeyframeEffect (see §3.1.3 Converting a CSS Keyframes rule into a KeyframeEffect), then stored against the name in the keyframes map.

Otherwise:

The change is ignored.

When the last @keyframes rule for a given name is removed from a style sheet, the action taken depends upon the current value stored against the name in the keyframes map.

If the current value has only been set due to style changes:

The value referenced by the @keyframes name is cleared from the keyframes map.

Otherwise:

The removal is ignored.

3.1.3. Converting a CSS Keyframes rule into a KeyframeEffect

A CSS @keyframes rule keyframes is converted into a KeyframeEffect using the following process:

1. Initialize an empty list keyframeList.
2. For each keyframe in keyframes:
   1. Create a new dictionary newKeyframe.
   2. Set newKeyframe’s offset to the offset defined by keyframe.
   3. If keyframe defines an animation-timing-function, set newKeyframe’s easing to the defined animation-timing-function.
   4. For each property defined in keyframe:
      1. Set newKeyframe[property] to the value associated with that property in keyframe
      2. Add property to referencedProperties.
3. Add newKeyframe to keyframeList.
4. Remove keyframes with duplicated offsets from keyframeListsuch that only the last keyframe to specify a given offset remains.
5. Construct a new KeyframeEffect, providing keyframeList as the frames parameter.

For CSS Animations, mappings into the Web Animations model are established pairwise between a specific AnimationEffect, the animating effect, and a specific element, the animating element.

When a CSS Animation is to be started, the value of animation-name is used as an index into the keyframes map, and the matching AnimationEffect (if any) is used to instantiate the animating effect by following the procedure defined below (see §3.4 Instantiating the animating effect ).

When a CSS Animation is started an Animation is constructed and associated with a Player on the Document Timeline. The animation is configured as follows:

• The target element is set to the animating element.
• The start delay is set to the current used value of the animation-delay property for the animating element.
• The iteration duration is set to the current used value of the animation-duration property for the animating element.
• The iteration count is set to the current used value of the animation-iteration-count property for the animating element.
• The direction is set to the current used value of the animation-direction property for the animating element.
• The fill mode is set to the current used value of the animation-fill-mode property for the animating element.
• The animation effect is set to the animating effect

Deal with animation-play-state

Explain where we need to cancel the Player for a css animation.

3.3. CSS Animations Events

The animationstart event is triggered to fire each time the corresponding Animation enters the active phase or after phase during a sample.

The animationiteration event is triggered to fire, on each sample where all of the following conditions are met:

1. the corresponding Animation is in the active phase
2. the corresponding Animation was in the active phase on the last sample
3. the corresponding Animation has a current iteration which is different to the current iteration of the last sample.

The animationend event is triggered to fire each time the corresponding Animation enters the after phase during a sample.

3.4. Instantiating the animating effect

The animating effect is instantiated from an AnimationEffect (the reference effect) stored in the keyframes attribute using the following process:

turn this into boring old specificese

If the reference effect is a KeyframeEffect which was converted from a CSS @keyframes rule using the process defined in §3.1.3 Converting a CSS Keyframes rule into a KeyframeEffect:

1. The result effect is created as a clone of the reference effect.
2. Convert "add" keyframes at an offset of 0 and 1 by snapshotting their referenced properties from the computed style of the animating element, then changing the keyframe to a "replace" keyframe.
3. Set the easing of all keyframes which do not have an easing to the value of animation-timing-function.

Otherwise

The animating effect is the reference effect.

4. Exposure in the Web Animations API

Players generated by CSS Animations and CSS Transitions are accessible via getAnimationPlayers with the restriction that source will point to a read-only TimedItem which disallows changes to its specified timing and effect.

Note: The Player remains fully functional. The source, playbackRate, currentTime, startTime and play control methods will be live and function as normal.

REMOVE ME: Definitions that should be resolved by Web Animations

animation player document timeline target element start delay iteration duration timing function animation effect keyframe animation effect keyframes after phase custom player priority cancel getanimationplayers TimedItem source specified timing effect playbackRate currentTime keyframeEffect offset animationEffect sample easing iteration count direction fill mode current iteration active phase