CSS Transitions Module Level 3

W3C Working Draft 01 December 2009

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Dean Jackson (Apple Inc)
David Hyatt (Apple Inc)
Chris Marrin (Apple Inc)
L. David Baron (Mozilla)


CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.

Status of this document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

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This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

The list of changes made to this specification is available.

Table of contents

1. Introduction

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.

2. Transitions

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 of left and background-color

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 list of values in transition properties do not have the same length, the list is repeated as a whole in order to provide necessary values.

Issue: Are the lists repeated to all be the length of the longest list, or are they repeated or truncated to match the length of the ‘transition-property’ list?


      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.

2.1. The transition-property Property

The transition-property property specifies the name of the CSS property to which the transition is applied.

We may ultimately want to support a keypath syntax for this property. A keypath syntax would enable different transitions to be specified for components of a property. For example the blur of a shadow could have a different transition than the color of a shadow.
Name: transition-property
Value: none | all | [ <IDENT> ] [ ‘,’ <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. A value of ‘all’ means that every property that is able to undergo a transition will do so. Otherwise, a list of properties to be transitioned is given.

We need to generate a list of properties that can be transitioned.
Is "none" even a useful value if the initial value is "all"? The syntax is more elegant if transition-duration defaults to 0 and this property defaults to "all", but another option is to default this property to "none" and duration to something reasonable, e.g., 250ms. This would force an author to specify transition-property in the shorthand all the time though.

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 ‘all’, ‘none’, ‘inherit’, and ‘initial’ allowed as items in a list of identifiers (of length greater than one)?

If one of the identifiers listed is a shorthand property, implementations must start transitions for any of its longhand sub-properties that are animatable, 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 or via a shorthand that contains it), then the transition that starts uses the duration, delay, and timing function at the index corresponding to the last occurrence of the property.

2.2. The transition-duration Property

The transition-duration property defines the length of time that a transition takes.

Name: transition-duration
Value: <time> [, <time>]*
Initial: 0
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 ‘0’, meaning that the transition is immediate (i.e. there will be no animation). A negative value for transition-duration is treated as ‘0’.

2.3. The 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.

The timing function is specified using a cubic bezier curve, which 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.

The timing function is a smooth curve from
   point P0 = (0,0) to point P3 = (1,1). The length and orientation of the
   line segment P0-P1 determines the tangent and the curvature of the curve
   at P0 and the line segment P2-P3 does the same at P3.

Timing Function Control Points

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.

Name: transition-timing-function
Value: ease | linear | ease-in | ease-out | ease-in-out | cubic-bezier(<number>, <number>, <number>, <number>) [, ease | linear | ease-in | ease-out | ease-in-out | 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.

The ease function is equivalent to cubic-bezier(0.25, 0.1, 0.25, 1.0).
The linear function is equivalent to cubic-bezier(0.0, 0.0, 1.0, 1.0).
The ease-in function is equivalent to cubic-bezier(0.42, 0, 1.0, 1.0).
The ease-out function is equivalent to cubic-bezier(0, 0, 0.58, 1.0).
The ease-in-out function is equivalent to cubic-bezier(0.42, 0, 0.58, 1.0)
Specifies a cubic-bezier curve. The four values specify points P1 and P2 of the curve as (x1, y1, x2, y2). All values must be in the range [0, 1] or the definition is invalid.

2.4. The 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 ‘0’ 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: 0
Applies to: all elements, :before and :after pseudo elements
Inherited: no
Percentages: N/A
Media: interactive
Computed value: Same as specified value.

2.5. The 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. eg. 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.

3. Starting of transitions

When the 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 of the change. Since this specification does not define what property changes 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.

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.

4. Automatically reversing transitions

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:

  1. Use the B and A states as its "from" and "to" states respectively. It does not use the current value as its from state, due to the rules below.
  2. Execute with the same duration T, but starting as if the transition had already begun, without any transition delay, at the moment which would cause the new transition to finish in TE from the moment of interruption. In other words, the new transition will execute as if it started T-TE in the past.
  3. Use a timing function that is the portion of the curve traversed up to the moment of interruption, followed in the opposite direction (towards the starting point). This will make the transition appear as if it is playing backwards.
  4. Ignore any transition delay.

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.

5. Transition Events

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.

Interface TransitionEvent

The TransitionEvent interface provides specific contextual information associated with transitions.

IDL Definition
  interface TransitionEvent : Event {
    readonly attribute DOMString          propertyName;
    readonly attribute float              elapsedTime;
    void               initTransitionEvent(in DOMString typeArg, 
                                          in boolean canBubbleArg, 
                                          in boolean cancelableArg, 
                                          in DOMString propertyNameArg,
                                          in float elapsedTimeArg);
propertyName of type DOMString, readonly
The name of the CSS property associated with the transition.
elapsedTime of type float, readonly
The amount of time the transition has been running, in seconds, when this event fired. Note that this value is not affected by the value of transition-delay.
The 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.
typeArg of type DOMString
Specifies the event type.
canBubbleArg of type boolean
Specifies whether or not the event can bubble.
cancelableArg of type boolean
Specifies whether or not the event's default action can be prevented. Since a TransitionEvent is purely for notification, there is no default action.
propertyNameArg of type DOMString
Specifies the name of the property associated with the Event
elapsedTimeArg of type float
Specifies the amount of time, in seconds, the transition has been running at the time of initialization.
No Return Value
No Exceptions

There is one type of transition event available.

The ‘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.

6. Animation of property types

The following describes how each property type undergoes transition or animation.

7. Animatable properties

7.1. Properties from CSS

Property Name Type
background-color color
background-image only gradients
background-position percentage, length
border-bottom-color color
border-bottom-width length
border-color color
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
border-width length
bottom length, percentage
color color
crop rectangle
font-size length, percentage
font-weight number
grid-* various
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 keywords, length, percentage
visibility visibility
width length, percentage
word-spacing length, percentage
z-index integer
zoom number

7.2. Properties from SVG

All properties defined as animatable in the SVG specification, provided they are one of the property types listed above.

8. References

Normative references

Dean Jackson; David Hyatt; Chris Marrin. CSS 2D Transforms Module Level 3. 20 March 2009. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2009/WD-css3-2d-transforms-20090320

Other references

Property index

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>]* 0 all elements, :before and :after pseudo elements no N/A interactive
transition-duration <time> [, <time>]* 0 all elements, :before and :after pseudo elements no N/A interactive
transition-property none | all | [ <IDENT> ] [ ‘,’ <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 | cubic-bezier(<number>, <number>, <number>, <number>) [, ease | linear | ease-in | ease-out | ease-in-out | cubic-bezier(<number>, <number>, <number>, <number>)]* ease all elements, :before and :after pseudo elements no N/A interactive