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

Changes to any of the animation properties defined in this specification cause the corresponding CSSAnimation object and its associated objects to be updated according to the correspondence between these properties and Web Animations concepts defined in § 3 Keyframes.

However, if the author modifies the animation using the Web Animations programming interface, the changes from the programming interface take precedence as follows:

• After a successful call to setKeyframes() on the KeyframeEffect associated with a CSSAnimation, any subsequent change to matching @keyframes rules or the resolved value of the animation-timing-function property for the target element will not be reflected in that animation.

  However, if the last matching @keyframes rule is removed the animation must still be canceled.

• After a successful call to updateTiming() on the KeyframeEffect associated with a CSSAnimation, for each property included in the timing parameter, any subsequent change to a corresponding animation property will not be reflected in that animation.

  For example, calling cssAnimation.effect.updateTiming({ duration: 1000 }) would cause subsequent changes to animation-duration to be ignored whilst changes to animation-delay would still be reflected in the KeyframeEffect's timing.

• After a successful call to play() or pause() on a CSSAnimation, any subsequent change to the animation-play-state will no longer cause the CSSAnimation to be played or paused as defined in § 3.5 The animation-play-state property.

• After a successful call to reverse() on a CSSAnimation or after successfully setting the startTime on a CSSAnimation, if, as a result of that call the play state of the CSSAnimation changes to or from the paused play state, any subsequent change to the animation-play-state will no longer cause the CSSAnimation to be played or paused as defined in § 3.5 The animation-play-state property.

  The requirement for a change to or from the paused play state ensures that even after calling reverse() or setting the startTime on a running animation, the animation continues to observe changes in animation-play-state.

• After successfully setting the effect of a CSSAnimation to null or some AnimationEffect other than the original KeyframeEffect, all subsequent changes to animation properties other than animation-name or animation-play-state will not be reflected in that animation. Similarly, any change to matching @keyframes rules will not be reflected in that animation. However, if the last matching @keyframes rule is removed the animation must still be canceled.

Note, the reference to a successful call in the above rules is necessary to ensure that when an exception is thrown by any of these methods, the override behavior is not applied.

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 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

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 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

   • ::marker

   • ::before

   • any other pseudo-elements not mentioned specifically in this list, sorted in ascending order by the Unicode codepoints that make up each selector

   • ::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.

3. Keyframes

For a given target (pseudo-)element, element, an animation name, name, and the position of the animation in element’s animation-name list, position, keyframe objects are generated as follows:

1. Let default timing function be the timing function at position position of the resolved value of the animation-timing-function for element, repeating the list as necessary as described in CSS Animations 1 § 3.2 The animation-name property.

2. Let default composite be replace.

3. Find the last @keyframes at-rule in document order with <keyframes-name> matching name.

   If there is no @keyframes at-rule with <keyframes-name> matching name, abort this procedure. In this case no animation is generated, and any existing animation matching name is canceled.

4. Let keyframes be an empty sequence of keyframe objects.

5. Let animated properties be an empty set of longhand CSS property names.

6. Perform a stable sort of the keyframe blocks in the @keyframes rule by the offset specified in the keyframe selector, and iterate over the result in reverse applying the following steps:

   1. Let keyframe offset be the value of the keyframe selector converted to a value in the range 0 ≤ keyframe offset ≤ 1.

   2. Let keyframe timing function be the value of the last valid declaration of animation-timing-function specified on the keyframe block, or, if there is no such valid declaration, default timing function.

   3. Let keyframe composite be the value of the last valid declaration of animation-composition specified on the keyframe block, or, if there is no such valid declaration, default composite.

   4. After converting keyframe timing function to its canonical form (e.g. such that step-end becomes steps(1, end)) let keyframe refer to the existing keyframe in keyframes with matching keyframe offset, timing function and composite, if any.

      If there is no such existing keyframe, let keyframe be a new empty keyframe with offset, keyframe offset, timing function, keyframe timing function, composite, keyframe composite, and prepend it to keyframes.

   5. Iterate over all declarations in the keyframe block and add them to keyframe such that:

      • Each shorthand property is expanded to its longhand subproperties.

      • All logical properties are converted to their equivalent physical properties.

      • For any expanded physical longhand properties that appear more than once, only the last declaration in source order is added.

        Note, since multiple keyframe blocks may specify the same keyframe offset, and since this algorithm iterates over these blocks in reverse, this implies that if any properties are encountered that have already added at this same keyframe offset, they should be skipped.

   6. Add each property name that was added to keyframe to animated properties.

7. If there is no keyframe in keyframes with offset 0, or if amongst the keyframes in keyframes with offset 0 not all of the properties in animated properties are present,

   1. Let initial keyframe be the keyframe in keyframes with offset 0, timing function default timing function and composite default composite.

      If there is no such keyframe, let initial keyframe be a new empty keyframe with offset 0, timing function default timing function, composite |default composite, and add it to keyframes after the last keyframe with offset 0.

   2. For each property in animated properties that is not present in some other keyframe with offset 0, add the computed value of that property for element to the keyframe.

8. Similarly, if there is no keyframe in keyframes with offset 1, or if amongst the keyframes in keyframes with offset 1 not all of the properties in animated properties are present,

   1. Let final keyframe be the keyframe in keyframes with offset 1, timing function default timing function and composite default composite.

      If there is no such keyframe, let final keyframe be a new empty keyframe with offset 1, timing function default timing function and composite default composite, and add it to keyframes after the last keyframe with offset 1.

   2. For each property in animated properties that is not present in some other keyframe with offset 1, add the computed value of that property for element to the keyframe.

The above procedure requires iterating over keyframe blocks in reverse. It could be rewritten so this is not required but that will likely change the behavior for some edge cases. We should verify what current implementations do and possible remove the requirement to iterate in reverse.

3.1. The animation-duration property

The animation-duration property specifies the iteration duration of the animation’s associated animation effect.

3.2. The animation-timing-function property

The animation-timing-function is used to determine the timing function applied to each keyframe as defined in § 3 Keyframes.

3.3. The animation-iteration-count property

The animation-iteration-count property specifies the iteration count of the animation’s associated animation effect.

3.4. The animation-direction property

The animation-direction property specifies the playback direction of the animation’s associated animation effect.

3.5. The animation-play-state property

The animation-play-state is used to pause or play the animation.

If at any time, including when the animation is first generated, the resolved value of animation-play-state corresponding to an animation is newly running, the implementation must run the procedure to play an animation for the given animation with the auto-rewind flag set to false.

If at any time, including when the animation is first generated, the resolved value of animation-play-state corresponding to an animation is newly paused, the implementation must run the procedure to pause an animation for the given animation.

The above requirements do not apply if the animation’s play state is being overridden by the Web Animations API as described in § 2 Animations.

3.6. The animation-delay property

The animation-delay property specifies the start delay of the animation’s associated animation effect.

3.7. The animation-fill-mode property

The animation-fill-mode property specifies the fill mode of the animation’s associated animation effect.

3.8. 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 brightness-pulse {
  0% {
    scale: 1.1;
    filter: brightness(130%);
  }
  10% {
    scale: 1;
    filter: brightness(100%);
  }
}

3.9. The animation-timeline property

The animation-timeline property defines the timeline used with the animation.

Note: This specification does not introduce any syntax to specify animation timelines but instead it is up to others specifications such as Scroll-linked Animations [SCROLL-ANIMATIONS] to do so.

<single-animation-timeline> = auto | none | <timeline-name>

The animation-timeline property is similar to properties like animation-name and animation-duration in that it can have one or more values, each one imparting additional behavior to a corresponding animation on the element, with the timelines matched up with animations as described here.

Each value has type <single-animation-timeline>, whose possible values have the following effects:

auto

The animation’s timeline is a DocumentTimeline, more specifically the default document timeline.

none

The animation is not associated with a timeline.

<timeline-name>

Find the last timeline at-rule in document order with its name matching <timeline-name>. If such a timeline at-rule exists, then the animation’s timeline is a timeline as defined by that rule. Otherwise the animation is not associated with a timeline.

<timeline-name> = <custom-ident> | <string>

Make it easier to use animation-name to select the timeline when animation-timeline is not specified. Allowing animation-name to be used for selecting timeline enables most common animations to have to use a single name for both their keyframes and timeline which is simple and ergonomics. The animation-timeline property gives authors additional control to independently select keyframes and timeline if necessary.

3.10. The animation shorthand property

The animation shorthand property syntax is as follows:

<single-animation> = <time> || <easing-function> || <time> || <single-animation-iteration-count> || <single-animation-direction> || <single-animation-fill-mode> || <single-animation-play-state> || [ none | <keyframes-name> ] || <single-animation-timeline>

4. Animation Events

4.1. 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].

The target for a CSS animation event is the animation’s owning element. If there is no owning element, no CSS animation events are dispatched (although the animation playback events defined in Web Animations are still dispatched at the corresponding CSSAnimation object).

For the purpose of determining which events to dispatch, the 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 associated effect. For example, a CSS Animation is in the before phase if its associated effect is in the before phase.

A CSS Animation that does not have an associated 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 associated effect.

For calculating the elapsedTime of each event, the following definitions are used:

• interval start = max(min(-start delay, active duration), 0)

• interval end = max(min(associated effect end - start delay, active duration), 0)

Each time a new animation frame is established and the animation does not have a pending play task or pending pause task, the events to dispatch are determined by comparing the animation’s phase before and after establishing the new animation frame as follows:

٭ 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 animation frame.

2. If previous current iteration is greater than current iteration, let iteration boundary be current iteration + 1, otherwise let it be current iteration.

3. 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 elapsedTime member of the AnimationEvent.

5. DOM Interfaces

5.1. The CSSAnimation interface

[Exposed=Window]
interface CSSAnimation : Animation {
  readonly attribute CSSOMString animationName;
};

✔MDN

CSSAnimation/animationName

In all current engines.

Firefox75+Safari13.1+Chrome84+

---

Opera?Edge84+

---

Edge (Legacy)?IENone

---

Firefox for Android?iOS Safari?Chrome for Android?Android WebView?Samsung Internet?Opera Mobile?

animationName, of type CSSOMString, readonly

The key used to find matching keyframes rules that define the associated 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.

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 or animation state 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();

elem.style.animation = 'fadeOut 1s paused';
const anim = elem.getAnimations()[0];
elem.style.animationPlayState = 'running';
console.log(anim.playState); // Should be 'running'.

6. Privacy and Security Considerations

This specification introduces no new privacy or security considerations.