Copyright © 2012 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.
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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.
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 style sheet 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 the 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.
If property is not specified for a keyframe, or is specified but invalid, the animation of that property proceeds as if that keyframe did not exist. Conceptually, it is as if a set of keyframes is constructed for each property that is present in any of the keyframes, and an animation is run independently for each property.
@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.
The following is the grammar for the keyframes rule.
keyframes_rule: KEYFRAMES_SYM S+ IDENT S* '{' S* keyframes_blocks '}' S*; keyframes_blocks: [ keyframe_selector '{' S* declaration? [ ';' S* declaration? ]* '}' S* ]* ; keyframe_selector: [ FROM_SYM | TO_SYM | PERCENTAGE ] S* [ ',' S* [ FROM_SYM | TO_SYM | PERCENTAGE ] S* ]*; @{K}{E}{Y}{F}{R}{A}{M}{E}{S} {return KEYFRAMES_SYM;} {F}{R}{O}{M} {return FROM_SYM;} {T}{O} {return TO_SYM;}
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 (i.e., between 0% and 25%) an "ease-out" timing function is used. Between the second and third keyframe (i.e., 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: | all elements, :before and :after pseudo 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: | all elements, :before and :after pseudo 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: | all elements, :before and :after pseudo 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 of 'animation-iteration-count'
are invalid. 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: | all elements, :before and :after pseudo 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 some or all cycles. 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 | reverse | alternate | alternate-reverse ] [, [ normal | reverse | alternate | alternate-reverse ] ]* |
Initial: | normal |
Applies to: | all elements, :before and :after pseudo 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: | all elements, :before and :after pseudo 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 ‘0s
’ 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: | all elements, :before and :after pseudo 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: | all elements, :before and :after pseudo 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. e.g. 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: | all elements, :before and :after pseudo 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
animation-name
property of the animation
that fired the event.
elapsedTime
of type
float
, readonly
animation-delay
, in which case the event
will be fired with an elapsedTime of (-1 * delay).
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:
animation-delay
then this event will fire once
the delay period has expired. A negative delay will cause the event to
fire with an elapsedTime equal to the absolute value of the delay.
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
Thanks especially to the feedback from Tab Atkins, Estelle Weyl, and all the rest of the www-style community.
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 | all elements, :before and :after pseudo elements | no | N/A | visual |
animation-delay | <time> [, <time>]* | 0s | all elements, :before and :after pseudo elements | no | N/A | visual |
animation-direction | [ normal | reverse | alternate | alternate-reverse ] [, [ normal | reverse | alternate | alternate-reverse ] ]* | normal | all elements, :before and :after pseudo elements | no | N/A | visual |
animation-duration | <time> [, <time>]* | 0s | all elements, :before and :after pseudo elements | no | N/A | visual |
animation-fill-mode | [ none | forwards | backwards | both ] [, [ none | forwards | backwards | both ] ]* | none | all elements, :before and :after pseudo elements | no | N/A | visual |
animation-iteration-count | [ infinite | <number> ] [, [ infinite | <number> ] ]* | 1 | all elements, :before and :after pseudo elements | no | N/A | visual |
animation-name | [ none | IDENT ] [, [ none | IDENT ] ]* | none | all elements, :before and :after pseudo elements | no | N/A | visual |
animation-play-state | [ running | paused ] [, [ running | paused ] ]* | running | all elements, :before and :after pseudo 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 | all elements, :before and :after pseudo elements | no | N/A | visual |