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://dom.spec.whatwg.org/; type: dfn; spec: dom
text: event target
spec:web-animations-1; type:dfn;
text:active duration
text:active phase; for:animation effect
text:active time
text:after phase; for:animation effect
text:animation class
text:animation effect
text:animation playback events
text:associated effect
text:associated effect end
text:before phase; for:animation effect
text:current iteration
text:current time; for:animation
text:composite operation
text:fill mode
text:idle play state
text:idle phase; for:animation effect
text:iteration duration
text:iteration count
text:iteration start
text:keyframe
text:pause an animation
text:paused play state
text:pending pause task
text:pending play task
text:play an animation
text:play state
text:playback direction
text:start delay
text:target element
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}
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 [[#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 {{KeyframeEffect/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 {{AnimationEffect/updateTiming()}}
on the {{KeyframeEffect}} associated with a {{CSSAnimation}},
for each property included in the
{{AnimationEffect/updateTiming(timing)/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 {{Animation/play()}} or {{Animation/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 [[#animation-play-state]].
* After a successful call to {{Animation/reverse()}} on a {{CSSAnimation}}
or after successfully setting the {{Animation/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 [[#animation-play-state]].
The requirement for a change to or from the [=paused play state=]
ensures that even after calling
{{Animation/reverse()}} or setting the {{Animation/startTime}}
on a running animation,
the animation continues to observe changes in 'animation-play-state'.
* After successfully setting the {{Animation/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.
## 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 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 [=associated 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
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 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
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.
# Keyframes # {#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#animation-name]].
1. Let |default composite| be ''replace''.
1. Find the last ''@keyframes'' at-rule in document order
with <> matching |name|.
If there is no ''@keyframes'' at-rule
with <> matching |name|,
abort this procedure.
In this case no animation is generated,
and any existing animation matching |name| is canceled.
1. Let |keyframes| be an empty sequence of [=keyframe=] objects.
1. Let |animated properties| be an empty set of longhand CSS property names.
1. 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.
1. 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|.
1. 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|.
1. 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|.
1. 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
[[css-writing-modes-4#logical-to-physical|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.
1. Add each property name that was added to |keyframe|
to |animated properties|.
1. 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.
1. 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.
1. 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.
1. 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.
Issue: 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.
## The 'animation-duration' property ## {#animation-duration}
The 'animation-duration' property specifies the [=iteration duration=]
of the animation's associated [=animation effect=].
## The 'animation-timing-function' property ## {#animation-timing-function}
The 'animation-timing-function' is used to determine the [=timing function=]
applied to each [=keyframe=] as defined in [[#keyframes]].
## The 'animation-iteration-count' property ## {#animation-iteration-count}
The 'animation-iteration-count' property specifies the [=iteration count=]
of the animation's associated [=animation effect=].
## The 'animation-direction' property ## {#animation-direction}
The 'animation-direction' property specifies the [=playback direction=]
of the animation's associated [=animation effect=].
## The 'animation-play-state' property ## {#animation-play-state}
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 [[#animations]].
## The 'animation-delay' property ## {#animation-delay}
The 'animation-delay' property specifies the [=start delay=]
of the animation's associated [=animation effect=].
## The 'animation-fill-mode' property ## {#animation-fill-mode}
The 'animation-fill-mode' property specifies the [=fill mode=]
of the animation's associated [=animation effect=].
## 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: no
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
targeting the 'scale' property.
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.
Issue: Create pictures of these examples and verify they make sense.
## The 'animation-timeline' property ## {#animation-timeline}
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.
Name: animation-timeline
Value: <>#
Initial: auto
Applies to: all elements
Inherited: no
Percentages: N/A
Computed value: list, each item either a case-sensitive [=css identifier=] or
the keywords ''single-animation-timeline/none'',
''single-animation-timeline/auto''.
Canonical order: per grammar
Animatable: no
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
[[css-animations-1#animation-name|here]].
Each value has type <>, 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=].
: <>
:: If a named [=scroll progress timeline=] or [=view progress timeline=]
is in scope on this element,
use the referenced timeline
as defined in [[scroll-animations-1#timeline-scope]].
Otherwise the animation is not associated with a [=timeline=].
: <>
:: Use the [=scroll progress timeline=] indicated by the given ''scroll()'' function.
See [[scroll-animations-1#scroll-notation]].
: <>
:: Use the [=view progress timeline=] indicated by the given ''view()'' function.
See [[scroll-animations-1#view-notation]].
Issue: 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.
## The 'animation' shorthand property ## {#animation-shorthand}
The 'animation' shorthand property syntax is as follows:
<single-animation> = <
# 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]].
The [=event target|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
[[web-animations-1#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 [=associated effect=].
For example, a CSS Animation is in the [=animation effect/before phase=] if its
[=associated effect=] is in the [=animation effect/before phase=].
A CSS Animation that does not have an [=associated effect=]
is considered to be in the idle phase
if its [=animation/current time=] is unresolved, in the
[=animation effect/before phase=] if its current time is less than zero,
and in the [=animation effect/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 {{AnimationEvent/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:
[=animation effect/active phase|active=] →
[=animation effect/active phase|active=]
and
the current iteration of the animation's [=associated effect=]
has changed since the previous animation frame
not [=animation effect/idle phase|idle=] and not
[=animation effect/after phase|after=] →
[=animation effect/idle phase|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 animation frame.
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}
: animationName
:: 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.
## 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 or animation state 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.