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).
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
2.2. 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:
-
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
-
::after
-
element children
-
-
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.
The interaction of these methods can be summarized as follows:
-
Calling
pause()
is sticky. The animation will remain paused until there is a subsequent call toplay()
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
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:
- 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:
Initial state | ||||||
---|---|---|---|---|---|---|
Event | A | B | C | D | E | |
Resulting state | play()
| A | B | A | B | B |
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.
3.2. The animation-composition property
The animation-composition property defines the composite operation used when multiple animations affect the same property simultaneously.
Name: | animation-composition |
---|---|
Value: | <single-animation-composition># |
Initial: | replace |
Applies to: | all elements |
Inherited: | no |
Percentages: | N/A |
Computed value: | list, each item a keyword as specified |
Canonical order: | per grammar |
Animation type: | not animatable |
<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; }
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%); } }
Create pictures of these examples and verify they make sense.
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].
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 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 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:
-
Let previous current iteration be the current iteration from the previous sample.
-
If previous current iteration is greater than current iteration, let iteration boundary be
current iteration + 1
, otherwise let it be current iteration. -
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 ; };
animationName
, of type CSSOMString, 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 theCSSAnimation
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 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
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
is initially updated, a user agent may defer recalculating
the computed value of the animation property.
However, the 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.
6. Privacy and Security Considerations
This specification introduces no new privacy or security considerations.