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CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.
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The list of changes made to this specification is available.
This section is not normative.
This document introduces new CSS features to enable implicit transitions, which describe how CSS properties can be made to change smoothly from one value to another over a given duration.
Normally when the value of a CSS property changes, the rendered result is instantly updated, with the affected elements immediately changing from the old property value to the new property value. This section describes a way to specify transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.
For example, suppose that transitions of one second have been defined
on the 'left'
and 'background-color'
properties. The following diagram
illustrates the effect of updating those properties on an element, in this
case moving it to the right and changing the background from red to blue.
This assumes other transition parameters still have their default values.
Transitions are a presentational effect. The computed value of a property transitions over time from the old value to the new value. Therefore if a script queries the computed style of a property as it is transitioning, it will see an intermediate value that represents the current animated value of the property.
Only animatable CSS properties can be transitioned. See the table at the end of this document for a list of properties that are animatable.
The transition for a property is defined using a number of new properties. For example:
div { transition-property: opacity; transition-duration: 2s; }The above example defines a transition on the
'opacity'
property that, when a new value is
assigned to it, will cause a smooth change between the old value and the
new value over a period of two seconds.Each of the transition properties accepts a comma-separated list, allowing multiple transitions to be defined, each acting on a different property. In this case, the individual transitions take their parameters from the same index in all the lists. For example:
div { transition-property: opacity, left; transition-duration: 2s, 4s; }This will cause the
'opacity'
property to
transition over a period of two seconds and the left property to
transition over a period of four seconds. In the case where the lists of values in transition
properties do not have the same length, the length of the ‘transition-property
’ list determines the
number of items in each list examined when starting transitions. The lists
are matched up from the first value: excess values at the end are not
used. If one of the other properties doesn't have enough comma-separated
values to match the number of values of ‘transition-property
’, the UA must
calculate its used value by repeating the list of values until there are
enough. This truncation or repetition does not affect the computed value.
Note: This is analogous to the behavior of the
‘background-*
’ properties, with
‘background-image
’ analogous to
‘transition-property
’.
div { transition-property: opacity, left, top, width; transition-duration: 2s, 1s; }The above example defines a transition on the
'opacity'
property of 2 seconds duration, a
transition on the 'left'
property of 1 second
duration, a transition on the 'top'
property
of 2 seconds duration and a transition on the 'width'
property of 1 second duration.'transition-property'
Property The 'transition-property'
property specifies the
name of the CSS property to which the transition is applied.
Name: | transition-property |
Value: | none | [ all | <IDENT> ] [ ‘, ’ [ all | <IDENT> ] ]*
|
Initial: | all |
Applies to: | all elements, :before and :after pseudo elements |
Inherited: | no |
Percentages: | N/A |
Media: | visual |
Computed value: | Same as specified value. |
A value of ‘none
’ means that no
property will transition. Otherwise, a list of properties to be
transitioned, or the keyword ‘all
’
which indicates that all properties are to be transitioned, is given.
If one of the identifiers listed is not a recognized property name or
is not an animatable property, the implementation must still start
transitions on the animatable properties in the list using the duration,
delay, and timing function at their respective indices in the lists for
‘transition-duration
’, ‘transition-delay
’, and ‘transition-timing-function
’. In other
words, unrecognized or non-animatable properties must be kept in the list
to preserve the matching of indices.
Are ‘none
’,
‘inherit
’, and ‘initial
’ allowed as items in a list of
identifiers (of length greater than one)?
For the keyword ‘all
’, or if
one of the identifiers listed is a shorthand property, implementations
must start transitions for any of its longhand sub-properties that are
animatable (or, for ‘all
’, all
animatable properties), using the duration, delay, and timing function at
the index corresponding to the shorthand.
If a property is specified multiple times in the value of ‘transition-property
’ (either on its own,
via a shorthand that contains it, or via the ‘all
’ value), then the transition that starts
uses the duration, delay, and timing function at the index corresponding
to the last item in the value of ‘transition-property
’ that calls for
animating that property.
Note: The all
value and
shorthand properties work in similar ways, so the all
value is just like a shorthand that covers all
properties.
'transition-duration'
Property The 'transition-duration'
property defines the
length of time that a transition takes.
Name: | transition-duration |
Value: | <time> [, <time>]* |
Initial: | 0s |
Applies to: | all elements, :before and :after pseudo elements |
Inherited: | no |
Percentages: | N/A |
Media: | interactive |
Computed value: | Same as specified value. |
This property specifies how long the transition from the old value to
the new value should take. By default the value is ‘0s
’, meaning that the transition is immediate (i.e.
there will be no animation). A negative value for transition-duration
is treated as ‘0s
’.
'transition-timing-function'
Property The 'transition-timing-function'
property describes
how the intermediate values used during a transition will be calculated.
It allows for a transition to change speed over its duration. These
effects are commonly called easing functions. In either case, a
mathematical function that provides a smooth curve is used.
Timing functions are either defined as a stepping function or a cubic Bézier curve. The timing function takes as its input the current elapsed percentage of the transition duration and outputs a percentage that determines how close the transition is to its goal state.
A stepping function is defined by a number that divides the domain of operation into equally sized intervals. Each subsequent interval is a equal step closer to the goal state. The function also specifies whether the change in output percentage happens at the start or end of the interval (in other words, if 0% on the input percentage is the point of initial change).
A cubic
Bézier curve is defined by four control points, P0
through P3 (see Figure 1). P0 and P3 are
always set to (0,0) and (1,1). The 'transition-timing-function'
property is used to
specify the values for points P1 and P2. These can
be set to preset values using the keywords listed below, or can be set to
specific values using the 'cubic-bezier'
function.
In the 'cubic-bezier'
function, P1 and
P2 are each specified by both an X and Y value.
Name: | transition-timing-function |
Value: | [ ease | linear | ease-in | ease-out | ease-in-out | step-start | step-end | steps(<integer>[, [ 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: | interactive |
Computed value: | Same as specified value. |
The timing functions have the following definitions.
start
’ or ‘end
’, and specifies the point at which the
change of values occur within the interval. If the second parameter is
omitted, it is given the value ‘end
’.
'transition-delay'
Property The 'transition-delay'
property defines when the
transition will start. It allows a transition to begin execution some some
period of time from when it is applied. A 'transition-delay'
value of ‘0s
’ means the transition will execute as soon as
the property is changed. Otherwise, the value specifies an offset from the
moment the property is changed, and the transition will delay execution by
that offset.
If the value for 'transition-delay'
is a negative time offset
then the transition will execute the moment the property is changed, but
will appear to have begun execution at the specified offset. That is, the
transition will appear to begin part-way through its play cycle. In the
case where a transition has implied starting values and a negative 'transition-delay'
, the starting values are
taken from the moment the property is changed.
Name: | transition-delay |
Value: | <time> [, <time>]* |
Initial: | 0s |
Applies to: | all elements, :before and :after pseudo elements |
Inherited: | no |
Percentages: | N/A |
Media: | interactive |
Computed value: | Same as specified value. |
'transition'
Shorthand Property The 'transition'
shorthand property combines the four properties described above 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 transition-duration. The second value that can be parsed as a time is assigned to transition-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: | transition |
Value: | [<‘transition-property ’> ||
<‘transition-duration ’> ||
<‘transition-timing-function ’> ||
<‘transition-delay ’> [, [<‘transition-property ’> ||
<‘transition-duration ’> ||
<‘transition-timing-function ’> ||
<‘transition-delay ’>]]*
|
Initial: | see individual properties |
Applies to: | all elements, :before and :after pseudo elements |
Inherited: | no |
Percentages: | N/A |
Media: | interactive |
Computed value: | Same as specified value. |
When the computed value of an animatable property changes,
implementations must decide what transitions to start based on the values
of the ‘transition-property
’, ‘transition-duration
’, ‘transition-timing-function
’, and ‘transition-delay
’ properties at the time
the animatable property would first have its new computed value.
This provides a way for authors to specify different values of the
‘transition-*
’ properties for the
“forward” and “reverse” transitions (but see below for special reversing behavior when an
incomplete transition is interrupted). Authors can specify the
value of ‘transition-duration
’, ‘transition-timing-function
’, or ‘transition-delay
’ in the same rule where
they specify the value that triggers the transition, or can change these
properties at the same time as they change the property that triggers the
transition. Since it's the new values of these ‘transition-*
’ properties that affect the
transition, these values will be used for the transitions to the
associated transitioning values. For example:
li { transition: background-color linear 1s; background: blue; } li:hover { background-color: green; transition-duration: 2s; /* applies to the transition *to* the :hover state */ }
When a list item with these style rules enters the :hover state, the
computed ‘transition-duration
’ at the time that
‘background-color
’ would have its
new value (‘green
’) is
‘2s
’, so the transition from
‘blue
’ to ‘green
’ takes 2 seconds. However, when the
list item leaves the :hover state, the transition from ‘green
’ to ‘blue
’ takes 1 second.
When the computed value of a property changes, implementations must
start transitions based on the relevant item (see the definition of ‘transition-property
’) in the computed
value of ‘transition-property
’. Corresponding to
this item there are values of ‘transition-duration
’ and ‘transition-delay
’ (see the rules on matching lists). Define the combined duration of the transition as the sum
of max(‘transition-duration
’, ‘0s
’) and ‘transition-delay
’. When the combined
duration is greater than ‘0s
’, then a
transition starts based on the values of ‘transition-duration
’, ‘transition-delay
’, and ‘transition-timing-function
’; in other
cases transitions do not occur.
Since this specification does not define when computed values change, and thus what changes to computed values are considered simultaneous, authors should be aware that changing any of the transition properties a small amount of time after making a change that might transition can result in behavior that varies between implementations, since the changes might be considered simultaneous in some implementations but not others.
Say something about simultaneity
Once the transition of a property has started, it must continue running
based on the original timing function, duration, and delay, even if the
‘transition-timing-function
’, ‘transition-duration
’, or ‘transition-delay
’ property changes before
the transition is complete. However, if the ‘transition-property
’ property changes such
that the transition would not have started, the transition must stop (and
the property must immediately change to its final value).
Implementations must not start a transition when the computed value of a property changes as a result of declarative animation (as opposed to scripted animation).
Implementations also must not start a transition when the computed value changes because it is inherited (directly or indirectly) from another element that is transitioning the same property.
A common type of transition effect is when a running transition is interrupted and the property is reset to its original value. An example is a hover effect on an element, where the pointer enters and exits the element before the effect has completed. If the outgoing and incoming transitions are executed using their specified durations and timing functions, the resulting effect can be distractingly asymmetric. Instead, the expected behavior is that the new transition should be the reverse of what has already executed.
If a running transition with duration T, executing so far for duration TE, from state A, to state B, is interrupted by a property change that would start a new transition back to state A, and all the transition attributes are the same (duration, delay and timing function), then the new transition must reverse the effect. The new transition must:
For example, suppose there is a transition with a duration of two seconds. If this transition is interrupted after 0.5 seconds and the property value assigned to the original value, then the new transition effect will be the reverse of the original, as if it had begun 1.5 seconds in the past.
Note that by using the defined from and to states for the reversing transition, it is also possible that it may reverse again, if interrupted; for example, if the transition reversing to state A was again interrupted by a property change to state B.
Issue: This introduces the concept of reversing a timing function, which the spec has otherwise resisted doing, and also introduces a discontinuity between transitions that have almost completed (which get automatically reversed and thus have their timing function reversed) and transitions that have fully completed (where the reversal doesn't lead to the timing function being reversed). An alternative proposal that avoids this is to follow the normal timing function algorithm, except multiply the duration (and also shorten any negative delay) by the (output) value of the transition timing function of the incomplete transition at the time it was interrupted, and, to account for multiple reverses in sequence, to divide by the shortening applied to the transition being interrupted. For more details see this thread: November 2009 part, December 2009 part, January 2010 part.
The completion of a CSS Transition generates a corresponding DOM Event. An event is fired for each property that undergoes a transition. This allows a content developer to perform actions that synchronize with the completion of a transition.
Each event provides the name of the property the transition is associated with as well as the duration of the transition.
The TransitionEvent
interface provides specific
contextual information associated with transitions.
interface TransitionEvent : Event { readonly attribute DOMString propertyName; readonly attribute float elapsedTime; readonly attribute DOMString pseudoElement; void initTransitionEvent(in DOMString typeArg, in boolean canBubbleArg, in boolean cancelableArg, in DOMString propertyNameArg, in float elapsedTimeArg, in DOMString pseudoElementArg); };
propertyName
of
type DOMString
, readonly
elapsedTime
of
type float
, readonly
transition-delay
.
pseudoElement
of
type DOMString
, readonly
initTransitionEvent
initTransitionEvent
method is
used to initialize the value of a TransitionEvent
created through the DocumentEvent
interface. This method may only be called before the
TransitionEvent
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.
Should new events being created still have init*Event methods?
typeArg
of type
DOMString
canBubbleArg
of type
boolean
cancelableArg
of type
boolean
propertyNameArg
of type
DOMString
Event
.
(See the propertyName
attribute.)
elapsedTimeArg
of type
float
pseudoElementArg
of type
DOMString
There is one type of transition event available.
transitionend
’ event
occurs at the completion of the transition. In the case where a
transition is removed before completion, such as if the
transition-property is removed, then the event will not fire.
The following describes how each property type undergoes transition or animation.
floor()
. This floor
behavior is inconsistent with SMIL Animation / SVG Animation.
visible
’, interpolated as a discrete step
where values of the timing function between 0 and 1 map to ‘visible
’ and other values of the timing
function (which occur only at the start/end of the transition or as a
result of ‘cubic-bezier()
’ functions
with Y values outside of [0, 1]) map to the closer endpoint; if neither
value is ‘visible
’ then not
interpolable.
'background-position'
is an example of a property
that would transition between lists of different lengths). If the
property does not allow extending its list, then no interpolation will
occur.
Issue: Need to describe handling of out-of-range values that can result from cubic-bezier(). Clamping values to the allowed range is probably the best solution.
For properties that exist at the time this specification was developed, this specification defines whether and how they are animated. However, future CSS specifications may define additional properties, additional values for existing properties, or additional animation behavior of existing values. In order to describe new animation behaviors and to have the definition of animation behavior in a more appropriate location, future CSS specifications should include an "Animatable:" line in the summary of the property's definition (in addition to the other lines described in [CSS21], section 1.4.2). This line should say "no" to indicate that a property cannot be animated or should reference an animation behavior (which may be one of the behaviors in the Animation of property types section above, or may be a new behavior) to define how the property animates. Such definitions override those given in this specification.
Property Name | Type |
---|---|
background-color | color |
background-position | percentage, length |
border-bottom-color | color |
border-bottom-width | length |
border-left-color | color |
border-left-width | length |
border-right-color | color |
border-right-width | length |
border-spacing | length |
border-top-color | color |
border-top-width | length |
bottom | length, percentage |
clip | rectangle |
color | color |
crop css3-content will likely advance slower than this specification, in which case this definition should move there | rectangle |
font-size | length, percentage |
font-weight | font weight |
height | length, percentage |
left | length, percentage |
letter-spacing | length |
line-height | number, length, percentage |
margin-bottom | length |
margin-left | length |
margin-right | length |
margin-top | length |
max-height | length, percentage |
max-width | length, percentage |
min-height | length, percentage |
min-width | length, percentage |
opacity | number |
outline-color | color |
outline-offset | integer |
outline-width | length |
padding-bottom | length |
padding-left | length |
padding-right | length |
padding-top | length |
right | length, percentage |
text-indent | length, percentage |
text-shadow | shadow |
top | length, percentage |
vertical-align | length, percentage |
visibility | visibility |
width | length, percentage |
word-spacing | length, percentage |
z-index | integer |
This list omits the following properties that Gecko can animate, and which likely should be included: background-size, border-*-radius, box-shadow, column-count, column-gap, column-rule-color, column-rule-width, column-width, font-size-adjust, font-stretch, marker-offset, text-decoration-color, transform, transform-origin.
All properties defined as animatable in the SVG specification, provided they are one of the property types listed above.
Thanks especially to the feedback from Tab Atkins, Aryeh Gregor, Vincent Hardy, Cameron McCormack, Alex Mogilevsky, and all the rest of the www-style community.
Property | Values | Initial | Applies to | Inh. | Percentages | Media |
---|---|---|---|---|---|---|
transition | [<‘transition-property’> || <‘transition-duration’> || <‘transition-timing-function’> || <‘transition-delay’> [, [<‘transition-property’> || <‘transition-duration’> || <‘transition-timing-function’> || <‘transition-delay’>]]* | see individual properties | all elements, :before and :after pseudo elements | no | N/A | interactive |
transition-delay | <time> [, <time>]* | 0s | all elements, :before and :after pseudo elements | no | N/A | interactive |
transition-duration | <time> [, <time>]* | 0s | all elements, :before and :after pseudo elements | no | N/A | interactive |
transition-property | none | [ all | <IDENT> ] [ ‘,’ [ all | <IDENT> ] ]* | all | all elements, :before and :after pseudo elements | no | N/A | visual |
transition-timing-function | [ ease | linear | ease-in | ease-out | ease-in-out | step-start | step-end | steps(<integer>[, [ 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 | interactive |