Copyright © 2011 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
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.
This is a public copy of the editors' draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don't cite this document other than as work in progress.
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This document was produced by the CSS Working Group (part of the Style Activity).
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animation-name
’ Property
animation-duration
’ Property
animation-timing-function
’ Property
animation-iteration-count
’ Property
animation-direction
’ Property
animation-play-state
’ Property
animation-delay
’ Property
animation-fill-mode
’ Property
animation
’ Shorthand Property
This section is not normative.
CSS Transitions [CSS3-TRANSITIONS] provide a way to interpolate CSS property values when they change as a result of underlying property changes. This provides an easy way to do simple animation, but the start and end states of the animation are controlled by the existing property values, and transitions provide little control to the author on how the animation progresses.
This proposal introduces defined animations, in which the author can specify the changes in CSS properties over time as a set of keyframes. Animations are similar to transitions in that they change the presentational value of CSS properties over time. The principal difference is that while transitions trigger implicitly when property values change, animations are explicitly executed when the animation properties are applied. Because of this, animations require explicit values for the properties being animated. These values are specified using animation keyframes, described below.
Many aspects of the animation can be controlled, including how many times the animation iterates, whether or not it alternates between the begin and end values, and whether or not the animation should be running or paused. An animation can also delay its start time.
CSS Animations affect computed property values. During the execution of an animation, the computed value for a property is controlled by the animation. This overrides the value specified in the normal styling system.
In the case of multiple animations specifying behavior for the same property, the animation defined last will override the previously defined animations.
An animation does not affect the computed value before the application of the animation, before the animation delay has expired, and after the end of the animation.
Computation of animated property values
The diagram above shows how property values are computed. The intrinsic style is shown at the top of the diagram. The computed value is derived from intrinsic style at the times when an animation is not running and also when an animation is delayed (see below for specification of animation delay). During an animation, the computed style is derived from the animated value.
The start time of an animation is the latter of two moments: the time at which the style is resolved that specifies the animation, or the time the document's load event is fired. Therefore, an animation specified in the document stylesheet will begin at the document load. An animation specified on an element by modifying the style after the document has loaded will start when the style is resolved. That may be immediately in the case of a pseudo style rule such as hover, or may be when the scripting engine returns control to the browser (in the case of style applied by script).
An animation applies to an element if the element has a value for ‘animation-name
’ that references a
valid keyframes rule. Once an animation has started it continues until it
ends or the ‘animation-name
’ is removed. The
values used for the keyframes and animation properties are snapshotted at
the time the animation starts. Changing them during the execution of the
animation has no effect. Note also, that changing the value of ‘animation-name
’ does not
necessarily restart an animation (e.g. if a list of animations are applied
and one is removed from the list, only that animation will stop; The other
animations will continue). In order to restart an animation, it must be
removed then reapplied.
The end of the animation is defined by the combination of the ‘animation-duration
’, ‘animation-iteration-count
’ and
‘animation-fill-mode
’ properties.
div { animation-name: diagonal-slide; animation-duration: 5s; animation-iteration-count: 10; } @keyframes diagonal-slide { from { left: 0; top: 0; } to { left: 100px; top: 100px; } }This will produce an animation that moves an element from (0, 0) to (100px, 100px) over five seconds and repeats itself nine times (for a total of ten iterations).
Keyframes are used to specify the values for the animating properties at various points during the animation. The keyframes specify the behavior of one cycle of the animation; the animation may iterate one or more times.
Keyframes are specified using a specialized CSS at-rule. A @keyframes rule consists of the keyword
"@keyframes", followed by an identifier giving a name for the animation
(which will be referenced using ‘animation-name
’), followed by a set
of style rules (delimited by curly braces).
The keyframe selector for a keyframe style rule consists of a
comma-separated list of percentage values or the keywords ‘from
’ or ‘to
’. The selector is used to specify the
percentage along the duration of the animation that the keyframe
represents. The keyframe itself is specified by the block of property
values declared on the selector. The keyword ‘from
’ is equivalent to the value 0%. The
keyword ‘to
’ is equivalent to the
value 100%. Note that the percentage unit specifier must be used on
percentage values. Therefore, "0" is an invalid keyframe selector.
If a 0% or "from" keyframe is not specified, then the user agent constructs a 0% keyframe using the computed values of the properties being animated. If a 100% or "to" keyframe is not specified, then the user agent constructs a 100% keyframe using the computed values of the properties being animated.
The keyframe declaration for a keyframe rule consists of properties and values. Properties that are unable to be animated are ignored in these rules, with the exception of animation-timing-function', the behavior of which is described below.
NOTE: describe what happens if a property is not present in all keyframes.
The @keyframes rule that is used by an animation will be the last one
encountered in sorted rules order that matches the name of the animation
specified by the ‘animation-name
’ property. @keyframes rules do not cascade; therefore an
animation will never derive keyframes from more than one @keyframes rule.
To determine the set of keyframes, all of the values in selectors are sorted in increasing order by time. If there are any duplicates, then the last keyframe specified inside the @keyframes rule will be used to provide the keyframe information for that time. There is no cascading within a @keyframes rule if multiple keyframes specify the same keyframe selector values.
@keyframes wobble { 0% { left: 100px; } 40% { left: 150px; } 60% { left: 75px; } 100% { left: 100px; } }Four keyframes are specified for the animation named "wobble". In the first keyframe, shown at the beginning of the animation cycle, the ‘
left
’ value of the animation is
100px. By 40% of the animation duration, ‘left
’ value has animated to 150px. At 60% of
the animation duration, the ‘left
’
value has animated back to 75px. At the end of the animation cycle, the
‘left
’ value has returned to
100px. The diagram below shows the state of the animation if it were given
a duration of 10s.
Animations states specified by keyframes
The following is the grammar for the keyframes rule.
keyframes-rule: '@keyframes' IDENT '{' keyframes-blocks '}'; keyframes-blocks: [ keyframe-selectors block ]* ; keyframe-selectors: [ 'from' | 'to' | PERCENTAGE ] [ ',' [ 'from' | 'to' | PERCENTAGE ] ]*;
A keyframe style rule may also declare the timing function that is to be used as the animation moves to the next keyframe.
@keyframes bounce { from { top: 100px; animation-timing-function: ease-out; } 25% { top: 50px; animation-timing-function: ease-in; } 50% { top: 100px; animation-timing-function: ease-out; } 75% { top: 75px; animation-timing-function: ease-in; } to { top: 100px; } }Five keyframes are specified for the animation named "bounce". Between the first and second keyframe (ie. between 0 and 25%) an "ease-out" timing function is used. Between the second and third keyframe (ie. between 25% and 50%) an "ease-in" timing function is used. And so on. The effect will appear as an element that moves up the page 50px, slowing down as it reaches its highest point then speeding up as it falls back to 100px. The second half of the animation behaves in a similar manner, but only moves the element 25px units up the page. A timing function specified on the "to" or 100% keyframe is ignored
See the ‘animation-timing-function
’ property
for more information.
animation-name
’ Property The ‘animation-name
’ property defines a
list of animations that apply. Each name is used to select the keyframe
at-rule that provides the property values for the animation. If the name
does not match any keyframe at-rule, there are no properties to be
animated and the animation will not execute. Furthermore, if the animation
name is ‘none
’ then there will be
no animation. This can be used to override any animations coming from the
cascade. If animations are attempting to modify the same property, then
the animation closest to the end of the list of names wins.
Each animation listed by name should have a corresponding value for the
other animation properties listed below. In the case where the other
properties do not have lists of the correct length, their values are
repeated to form a list with the same number of entries as ‘animation-name
’.
Name: | animation-name |
Value: | none | IDENT [, none | IDENT ]* |
Initial: | none |
Applies to: | block-level and inline-level elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | Same as specified value. |
animation-duration
’ Property The ‘animation-duration
’ property
defines the length of time that an animation takes to complete one cycle.
Name: | animation-duration |
Value: | <time> [, <time>]* |
Initial: | 0s |
Applies to: | block-level and inline-level elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | Same as specified value. |
By default the value is ‘0s
’,
meaning that the animation cycle is immediate. A negative value for animation-duration is
treated as ‘0s
’. In this case animation-fill-mode still
applies, so an animation that fills backwards will show the value of the
0% keyframe during any delay period, and an animation that fill forwards
will retain the value specified at the 100% keyframe, even if the
animation was instantaneous. Also, animation events are still fired.
animation-timing-function
’ Property
The ‘animation-timing-function
’ property
describes how the animation will progress over one cycle of its duration.
See the ‘transition-timing-function
’ property [CSS3-TRANSITIONS] for a
complete description of timing function calculation.
Name: | animation-timing-function |
Value: | ease | linear | ease-in | ease-out | ease-in-out | step-start | step-end | steps(<number>[, start | end ]) | cubic-bezier(<number>, <number>, <number>, <number>) [, ease | linear | ease-in | ease-out | ease-in-out | step-start | step-end | steps(<number>[, start | end ]) | cubic-bezier(<number>, <number>, <number>, <number>)]* |
Initial: | ease |
Applies to: | block-level and inline-level elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | Same as specified value. |
For a keyframed animation, the ‘animation-timing-function
’ applies
between keyframes, not over the entire animation. For example, in the case
of an ease-in-out timing function, an animation will ease in at the start
of the keyframe and ease out at the end of the keyframe. A ‘animation-timing-function
’ defined
within a keyframe block applies to that keyframe, otherwise the timing
function specified for the animation is used.
animation-iteration-count
’ Property
The ‘animation-iteration-count
’ property
defines the number of times an animation cycle is played. The default
value is one, meaning the animation will play from beginning to end once.
A value of ‘infinite
’ will cause the animation to
repeat forever. Non-integer numbers will cause the animation to end
part-way through a cycle. Negative values for ‘animation-iteration-count
’ are
treated as zero. This property is often used with an ‘animation-direction
’ value of ‘alternate
’, which will cause the
animation to play in reverse on alternate cycles.
Name: | animation-iteration-count |
Value: | infinite | <number> [, infinite | <number>]* |
Initial: | 1 |
Applies to: | block-level and inline-level elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | Same as specified value. |
animation-direction
’ Property The ‘animation-direction
’ property
defines whether or not the animation should play in reverse on alternate
cycles. If ‘alternate
’ is specified, the animation
cycle iterations that are odd counts are played in the normal direction,
and the animation cycle iterations that are even counts are played in a
reverse direction. When an animation is played in reverse the timing
functions are also reversed. For example, when played in reverse an
ease-in animation would appear to be an ease-out animation.
Name: | animation-direction |
Value: | normal | alternate [, normal | alternate]* |
Initial: | normal |
Applies to: | block-level and inline-level elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | Same as specified value. |
animation-play-state
’ Property The ‘animation-play-state
’ property
defines whether the animation is running or paused. A running animation
can be paused by setting this property to ‘paused
’.
To continue running a paused animation this property can be set to ‘running
’.
A paused animation will continue to display the current value of the
animation in a static state, as if the time of the animation is constant.
When a paused animation is resumed, it restarts from the current value,
not necessarily from the beginning of the animation.
Name: | animation-play-state |
Value: | running | paused [, running | paused]* |
Initial: | running |
Applies to: | block-level and inline-level elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | Same as specified value. |
animation-delay
’ Property The ‘animation-delay
’ property defines
when the animation will start. It allows an animation to begin execution
some time after it is applied. An ‘animation-delay
’ value of
‘0
’ means the animation will execute as
soon as it is applied. Otherwise, the value specifies an offset from the
moment the animation is applied, and the animation will delay execution by
that offset.
If the value for ‘animation-delay
’ is a negative time
offset then the animation will execute the moment it is applied, but will
appear to have begun execution at the specified offset. That is, the
animation will appear to begin part-way through its play cycle. In the
case where an animation has implied starting values and a negative ‘animation-delay
’, the starting
values are taken from the moment the animation is applied.
Name: | animation-delay |
Value: | <time> [, <time>]* |
Initial: | 0s |
Applies to: | block-level and inline-level elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | Same as specified value. |
animation-fill-mode
’ Property The ‘animation-fill-mode
’ property
defines what values are applied by the animation outside the time it is
executing. By default, an animation will not affect property values
between the time it is applied (the ‘animation-name
’ property is set on
an element) and the time it begins execution (which is determined by the
‘animation-delay
’ property). Also,
by default an animation does not affect property values after the
animation ends (determined by the ‘animation-duration
’ property). The
value of ‘animation-fill-mode
’ can override
this behavior.
If the value for ‘animation-fill-mode
’ is ‘backwards
’, then the animation will
apply the property values defined in its 0% or ‘from
’ keyframe as soon as the animation is
applied, during the period defined by ‘animation-delay
’.
If the value for ‘animation-fill-mode
’ is ‘forwards
’, then the animation will
apply the property values defined in its last executing keyframe after the
final iteration of the animation, until the animation style is removed.
The last executing keyframe is the ‘to
’ or ‘100%
’ keyframe, unless the animation has ‘animation-direction
’ set to ‘alternate
’ and both a finite and even
iteration count, in which case it is the ‘from
’ or ‘0%
’ keyframe.
If the value for ‘animation-fill-mode
’ is ‘both
’,
then the animation will follow the rules for both ‘forwards
’ and ‘backwards
’. That is, it will extend the
animation properties in both directions.
Name: | animation-fill-mode |
Value: | none | forwards | backwards | both [, none | forwards | backwards | both]* |
Initial: | none |
Applies to: | block-level and inline-level elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | Same as specified value. |
animation
’ Shorthand Property The ‘animation
’ shorthand property
combines seven of the animation properties into a single property.
Note that order is important in this property. The first value that can be parsed as a time is assigned to the animation-duration. The second value that can be parsed as a time is assigned to animation-delay.
An alternative proposal is to accept the font shorthand approach of using a "/" character between the values of the same type. eg. 2s/4s would mean a duration of 2 seconds and a delay of 4 seconds.
Name: | animation |
Value: | [<animation-name> || <animation-duration> || <animation-timing-function> || <animation-delay> || <animation-iteration-count> || <animation-direction> || <animation-fill-mode>] [, [<animation-name> || <animation-duration> || <animation-timing-function> || <animation-delay> || <animation-iteration-count> || <animation-direction> || <animation-fill-mode>] ]* |
Initial: | see individual properties |
Applies to: | block-level and inline-level elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | Same as specified value. |
Several animation related events are available through the DOM Event system. The start and end of an animation, and the end of each iteration of an animation all generate DOM events. An element can have multiple properties being animated simultaneously. This can occur either with a single animation-name value with keyframes containing multiple properties, or with multiple animation-name values. For the purposes of events, each animation-name specifies a single animation. Therefore an event will be generated for each animation-name value and not necessarily for each property being animated.
The time the animation has been running is sent with each event
generated. This allows the event handler to determine the current
iteration of a looping animation or the current position of an alternating
animation. This time does not include any time the animation was in the
paused
play state.
The AnimationEvent
interface provides specific
contextual information associated with Animation events.
interface AnimationEvent : Event { readonly attribute DOMString animationName; readonly attribute float elapsedTime; void initAnimationEvent(in DOMString typeArg, in boolean canBubbleArg, in boolean cancelableArg, in DOMString animationNameArg, in float elapsedTimeArg); };
animationName
of
type DOMString
, readonly
elapsedTime
of type
float
, readonly
initAnimationEvent
initAnimationEvent
method is used
to initialize the value of an AnimationEvent
created
through the DocumentEvent
interface. This method may only be called before the
AnimationEvent
has been dispatched via the
dispatchEvent
method, though it may be called multiple
times during that phase if necessary. If called multiple times, the
final invocation takes precedence.
typeArg
of type
DOMString
canBubbleArg
of type
boolean
cancelableArg
of type
boolean
animationNameArg
of type
DOMString
Event
‘s animation name.
elapsedTimeArg
of type
float
The different types of Animation events that can occur are:
animationend
’ event
occurs when the animation finishes.
animationiteration
’
event occurs at the end of each iteration of an animation for which animation-iteration-count
is greater than one. This event does not occur for animations with an
iteration count of one.
CSS animation is exposed to the CSSOM through a pair of new interfaces describing the keyframes.
The following 2 rule types are added to the CSSRule
interface. They provide identification for the new keyframe and
keyframes rules.
interface CSSRule { ... const unsigned short KEYFRAMES_RULE = 7; const unsigned short KEYFRAME_RULE = 8; ... };
The CSSKeyframeRule
interface represents the style rule
for a single key.
interface CSSKeyframeRule : CSSRule { attribute DOMString keyText; readonly attribute CSSStyleDeclaration style; };
keyText
of type
DOMString
from
this value will be 0, and if the value in
the CSS style is to
this value will be 1.style
of type
CSSStyleDeclaration
The CSSKeyframesRule
interface represents a complete set
of keyframes for a single animation.
interface CSSKeyframesRule : CSSRule { attribute DOMString name; readonly attribute CSSRuleList cssRules; void appendRule(in DOMString rule); void deleteRule(in DOMString key); CSSKeyframeRule findRule(in DOMString key); };
name
of type
DOMString
animation-name
property.cssRules
of type
CSSRuleList
appendRule
appendRule
method appends the
passed CSSKeyframeRule into the list at the passed key.
rule
of type
DOMString
@keyframes
rule.
deleteRule
deleteRule
method deletes the
CSSKeyframeRule with the passed key. If a rule with this key does
not exist, the method does nothing.
key
of type
DOMString
findRule
findRule
method returns the rule
with a key matching the passed key. If no such rule exists, a null
value is returned.
key
of type
DOMString
CSSKeyframeRule
Property | Values | Initial | Applies to | Inh. | Percentages | Media |
---|---|---|---|---|---|---|
animation | [<animation-name> || <animation-duration> || <animation-timing-function> || <animation-delay> || <animation-iteration-count> || <animation-direction> || <animation-fill-mode>] [, [<animation-name> || <animation-duration> || <animation-timing-function> || <animation-delay> || <animation-iteration-count> || <animation-direction> || <animation-fill-mode>] ]* | see individual properties | block-level and inline-level elements | no | N/A | visual |
animation-delay | <time> [, <time>]* | 0s | block-level and inline-level elements | no | N/A | visual |
animation-direction | normal | alternate [, normal | alternate]* | normal | block-level and inline-level elements | no | N/A | visual |
animation-duration | <time> [, <time>]* | 0s | block-level and inline-level elements | no | N/A | visual |
animation-fill-mode | none | forwards | backwards | both [, none | forwards | backwards | both]* | none | block-level and inline-level elements | no | N/A | visual |
animation-iteration-count | infinite | <number> [, infinite | <number>]* | 1 | block-level and inline-level elements | no | N/A | visual |
animation-name | none | IDENT [, none | IDENT ]* | none | block-level and inline-level elements | no | N/A | visual |
animation-play-state | running | paused [, running | paused]* | running | block-level and inline-level elements | no | N/A | visual |
animation-timing-function | ease | linear | ease-in | ease-out | ease-in-out | step-start | step-end | steps(<number>[, start | end ]) | cubic-bezier(<number>, <number>, <number>, <number>) [, ease | linear | ease-in | ease-out | ease-in-out | step-start | step-end | steps(<number>[, start | end ]) | cubic-bezier(<number>, <number>, <number>, <number>)]* | ease | block-level and inline-level elements | no | N/A | visual |