CSS Transitions

Abstract

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/.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

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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
• 2. Transitions
  • 2.1. The ‘transition-property’ Property
  • 2.2. The ‘transition-duration’ Property
  • 2.3. The ‘transition-timing-function’ Property
  • 2.4. The ‘transition-delay’ Property
  • 2.5. The ‘transition’ Shorthand Property
• 3. Starting of transitions
• 4. Automatically reversing interrupted transitions
• 5. Transition Events
• 6. Animation of property types
• 7. Animatable properties
  • 7.1. Properties from CSS
  • 7.2. Properties from SVG
• 8. Acknowledgments
• 9. References
  • Normative references
  • Other references
• Property index
• Index

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;
  }
  

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;
  }

  

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;
      }
      

While authors can use transitions to create dynamically changing content, dynamically changing content can lead to seizures in some users. For information on how to avoid content that can lead to seizures, see Guideline 2.3: Seizures: Do not design content in a way that is known to cause seizures ([WCAG20]).

2.1. The ‘transition-property’ Property

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

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.

The keywords ‘none’, ‘inherit’, and ‘initial’ are not permitted as items within a list of more that one identifier; any list that uses them is syntactically invalid. In other words, the <IDENT> production in <single-transition-property> matches any identifier other than these three keywords.

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 ‘all’ shorthand property work in similar ways, so the ‘all’ value is just like a shorthand that covers all properties.

2.2. The ‘transition-duration’ Property

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

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’ renders the declaration invalid.

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.

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 the percentage of the way the transition is from its start value to its end value. How this output is used is defined by the interpolation rules for the value type.

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).

Step timing functions

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.

Bézier Timing Function Control Points

The timing functions have the following definitions.

ease

The ease function is equivalent to cubic-bezier(0.25, 0.1, 0.25, 1).

linear

The linear function is equivalent to cubic-bezier(0, 0, 1, 1).

ease-in

The ease-in function is equivalent to cubic-bezier(0.42, 0, 1, 1).

ease-out

The ease-out function is equivalent to cubic-bezier(0, 0, 0.58, 1).

ease-in-out

The ease-in-out function is equivalent to cubic-bezier(0.42, 0, 0.58, 1)

step-start

The step-start function is equivalent to steps(1, start).

step-end

The step-end function is equivalent to steps(1, end).

steps(<integer>[, [ start | end ] ]?)

Specifies a stepping function, described above, taking two parameters. The first parameter specifies the number of intervals in the function. It must be a positive integer (greater than 0). The second parameter, which is optional, is either the value ‘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’.

cubic-bezier(<number>, <number>, <number>, <number>)

Specifies a cubic-bezier curve. The four values specify points P₁ and P₂ of the curve as (x1, y1, x2, y2). Both x values must be in the range [0, 1] or the definition is invalid. The y values can exceed this range.

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 ‘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.

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 within the items in this property: the first value that can be parsed as a time is assigned to the transition-duration, and the second value that can be parsed as a time is assigned to transition-delay.

If there is more than one <single-transition> in the shorthand, and any of the transitions has ‘none’ as the <single-transition-property>, then the declaration is invalid.

3. Starting of transitions

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.

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 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 computed 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.

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 interrupted 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.

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.

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

  [Constructor(DOMString type, optional TransitionEventInit eventInitDict)]
  interface TransitionEvent : Event {
    readonly attribute DOMString          propertyName;
    readonly attribute float              elapsedTime;
    readonly attribute DOMString          pseudoElement;
  };

  dictionary AnimationEventInit : EventInit {
    DOMString propertyName;
    float elapsedTime;
    DOMString pseudoElement;
  }
  

Attributes

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.

pseudoElement of type DOMString, readonly

The name (beginning with two colons) of the CSS pseudo-element on which the transition occured (in which case the target of the event is that pseudo-element's corresponding element), or the empty string if the transition occurred on an element (which means the target of the event is that element).

TransitionEvent(type, animEventDict) is an event constructor.

There is one type of transition event available.

transitionend

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.

• Bubbles: Yes
• Cancelable: Yes
• Context Info: propertyName, elapsedTime, pseudoElement

6. Animation of property types

When interpolating between two values, V_start and V_end, interpolation is done using the output p of the timing function, which gives the portion of the value space that the interpolation has crossed. Thus the result of the interpolation is V_res = (1 - p) ⋅ V_start + p ⋅ V_end.

However, if this value (V_res) is outside the allowed range of values for the property, then it is clamped to that range. This can occur if p is outside of the range 0 to 1, which can occur if a timing function is specified with a y1 or y2 that is outside the range 0 to 1.

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

• color: interpolated via red, green, blue and alpha components (treating each as a number, see below). The interpolation is done between premultiplied colors (that is, colors for which the red, green, and blue components specified have been multiplied by the alpha).
• length: interpolated as real numbers.
• percentage: interpolated as real numbers.
• length, percentage, or calc: when both values are lengths, interpolated as lengths; when both values are percentages, interpolated as percentages; otherwise, both values are converted into a ‘calc()’ function that is the sum of a length and a percentage (each possibly zero), and these ‘calc()’ functions have each half interpolated as real numbers.
• integer: interpolated via discrete steps (whole numbers). The interpolation happens in real number space and is converted to an integer by rounding to the nearest integer, with values halfway between a pair of integers rounded towards positive infinity.
• font weight: interpolated via discrete steps (multiples of 100). The interpolation happens in real number space and is converted to an integer by rounding to the nearest multiple of 100, with values halfway between multiples of 100 rounded towards positive infinity.
• number: interpolated as real (floating point) numbers.
• rectangle: interpolated via the x, y, width and height components (treating each as a number).
• visibility: if one of the values is ‘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.
• shadow list: Each shadow in the list is interpolated via the color (as color) component, and x, y, blur, and (when appropriate) spread (as length) components. For each shadow, if one input shadow is ‘inset’ and the other is not, then the result for that shadow matches the inputs; otherwise the entire list is not interpolable. If the lists of shadows have different lengths, then the shorter list is padded at the end with shadows whose color is ‘transparent’, all lengths are ‘0’, and whose ‘inset’ (or not) matches the longer list.
• gradient: interpolated via the positions and colors of each stop. They must have the same type (radial or linear) and same number of stops in order to be animated. Note: [CSS3-IMAGES] may extend this definition.
• paint server (SVG): interpolation is only supported between: gradient to gradient and color to color. They then work as above.
• simple list of other types: If the lists have the same number of items, and each pair of values can be interpolated, each item in the list is interpolated using the rules given for those types. Otherwise the values are not interpolable.
• repeatable list of other types: The result list has a length that is the least common multiple of the lengths of the input lists. Each item in the result is the interpolation of the value from each input list repeated to the length of the result list. If a pair of values cannot be interpolated, then the lists are not interpolable. The repeatable list concept ensures that a list that is conceptually repeated to a certain length (as ‘background-origin’ is repeated to the length of the ‘background-image’ list) or repeated infinitely will smoothly transition between any values, and so that the computed value will properly represent the result (and potentially be inherited correctly).

Future specifications may define additional types that can be animated.

See the definition of ‘transition-property’ for how animation of shorthand properties and the ‘all’ value is applied to any properties (in the shorthand) that can be animated.

7. Animatable properties

The definition of each CSS property defines when the values of that property can be interpolated by referring to the definitions of property types in the previous section. Values are animatable when both the from and the to values of the property have the type described. (When a composite type such as "length, percentage, or calc" is listed, this means that both values must fit into that composite type.) When multiple types are listed in the form "either A or B", both values must be of the same type to be interpolable.

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.

7.1. Properties from CSS

The following definitions define the animation behavior for properties in CSS Level 2 Revision 1 ([CSS21]) and in Level 3 of the CSS Color Module ([CSS3COLOR]).

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. Acknowledgments

Thanks especially to the feedback from Tab Atkins, Carine Bournez, Aryeh Gregor, Vincent Hardy, Cameron McCormack, Alex Mogilevsky, and all the rest of the www-style community.

9. References

Normative references

Other references

[CSS21]

Bert Bos; et al. Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-CSS2-20110607/

[CSS3-IMAGES]

Elika J. Etemad; Tab Atkins Jr. CSS Image Values and Replaced Content Module Level 3. 17 April 2012. W3C Candidate Recommendation. (Work in progress.) URL: http://www.w3.org/TR/2012/CR-css3-images-20120417/

[CSS3COLOR]

Tantek Çelik; Chris Lilley; L. David Baron. CSS Color Module Level 3. 7 June 2011. W3C Recommendation. URL: http://www.w3.org/TR/2011/REC-css3-color-20110607

[WCAG20]

Ben Caldwell; et al. Web Content Accessibility Guidelines (WCAG) 2.0. 11 December 2008. W3C Recommendation. URL: http://www.w3.org/TR/2008/REC-WCAG20-20081211/

Property index

Index

• combined duration, 3.
• <single-transition>, 2.5.
• <single-transition-property>, 2.1.
• <single-transition-timing-function>, 2.3.
• transition, 2.5.
• transition-delay, 2.4.
• transition-duration, 2.2.
• transitionend, 5.
• TransitionEvent, 5.
  • elapsedTime, 5.
  • propertyName, 5.
  • pseudoElement, 5.
• transition-property, 2.1.
• transition-timing-function, 2.3.