CSS Transitions Level 1

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.

1.1. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

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 gradual 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 value of a property (or other data depending on it) as it is transitioning, it will see an intermediate value that represents the current animated value of the property.

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 user agent 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, 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 properties must be kept in the list to preserve the matching of indices.

The <custom-ident> production in <single-transition-property> also excludes the keyword none, in addition to the keywords always excluded from <custom-ident>. This means that 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.

For the keyword all, or if one of the identifiers listed is a shorthand property, implementations must start transitions for all its longhand sub-properties (or, for all, all 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.

Timing functions are defined in the separate CSS Easing Functions module [css-easing-1]. The input progress value used is the percentage of the transition duration, and the output progress value is used as the p value when interpolating the property value (see § 4 Application of transitions).

2.4. The transition-delay Property

The transition-delay property defines when the transition will start. It allows a transition to begin execution after 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

Implementations must maintain a set of running transitions, each of which applies to a specific element and non-shorthand property. Each of these transitions also has a start time, end time, start value, end value, reversing-adjusted start value, and reversing shortening factor. Transitions are added to this set as described in this section, and are removed from this set when they complete or when implementations are required to cancel them. For the rationale behind the reversing-adjusted start value and reversing shortening factor, see § 3.1 Faster reversing of interrupted transitions.

Implementations must also maintain a set of completed transitions, each of which (like running transitions) applies to a specific element and non-shorthand property. This specification maintains the invariant that there is never both a running transition and a completed transition for the same property and element.

If an element is no longer in the document, implementations must cancel any running transitions on it and remove transitions on it from the completed transitions.

Various things can cause the computed values of properties on an element to change. These include insertion and removal of elements from the document tree (which both changes whether those elements have computed values and can change the styles of other elements through selector matching), changes to the document tree that cause changes to which selectors match elements, changes to style sheets or style attributes, and other things. This specification does not define when computed values are updated, beyond saying that implementations must not use, present, or display something resulting from the CSS cascading, value computation, and inheritance process [CSS3CASCADE] without updating the computed value (which means merely that implementations cannot avoid meeting requirements of this specification by claiming not to have updated the computed value as part of handling a style change). However, when an implementation updates the computed value of a property on an element to reflect one of these changes, or computes the computed value of a property on an element newly added to the document, it must update the computed value for all properties and elements to reflect all of these changes at the same time (or at least it must be undetectable that it was done at a different time). This processing of a set of simultaneous style changes is called a style change event. (Implementations typically have a style change event to correspond with their desired screen refresh rate, and when up-to-date computed style or layout information is needed for a script API that depends on it.)

Since this specification does not define when a style change event occurs, 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.

When a style change event occurs, implementations must start transitions based on the computed values that changed in that event. If an element is not in the document during that style change event or was not in the document during the previous style change event, then transitions are not started for that element in that style change event. Otherwise, define the before-change style as the computed values of all properties on the element as of the previous style change event, except with any styles derived from declarative animations such as CSS Transitions, CSS Animations ([CSS3-ANIMATIONS]), and SMIL Animations ([SMIL-ANIMATION], [SVG11]) updated to the current time. Likewise, define the after-change style as the computed values of all properties on the element based on the information known at the start of that style change event, but using the computed values of the animation-* properties from the before-change style, excluding any styles from CSS Transitions in the computation, and inheriting from the after-change style of the parent. Note that this means the after-change style does not differ from the before-change style due to newly created or canceled CSS Animations.

For each element with a before-change style and an after-change style, and each property (other than shorthands), define the matching transition-property value as the last value in the transition-property in the element’s after-change style that matches the property, as described in § 2.1 The transition-property Property. If there is such a value, then corresponding to it, there is a matching transition duration, a matching transition delay, and a matching transition timing function in the values in the after-change style of transition-duration, transition-delay, and transition-timing-function (see the rules on matching lists). Define the combined duration of the transition as the sum of max(matching transition duration, 0s) and the matching transition delay.

When comparing the before-change style and after-change style for a given property, the property values are transitionable if they have an animation type that is neither not animatable nor discrete.

Note: Even if a property is defined to have an animation type that is not discrete, for a particular pair of property values the animation type may be discrete. For example, the animation type of the box-shadow property is shadow list, which defines that when the inset keyword is absent in one value but present in the other, discrete animation is used. As a result 0px 0px black and inset 10px 10px black are not transitionable.

For each element and property, the implementation must act as follows:

1. If all of the following are true:
   • the element does not have a running transition for the property,
   • the before-change style is different from the after-change style for that property, and the values for the property are transitionable,
   • the element does not have a completed transition for the property or the end value of the completed transition is different from the after-change style for the property,
   • there is a matching transition-property value, and
   • the combined duration is greater than 0s,
   then implementations must remove the completed transition (if present) from the set of completed transitions and start a transition whose:
   • start time is the time of the style change event plus the matching transition delay,
   • end time is the start time plus the matching transition duration,
   • start value is the value of the transitioning property in the before-change style,
   • end value is the value of the transitioning property in the after-change style,
   • reversing-adjusted start value is the same as the start value, and
   • reversing shortening factor is 1.
2. Otherwise, if the element has a completed transition for the property and the end value of the completed transition is different from the after-change style for the property, then implementations must remove the completed transition from the set of completed transitions.
3. If the element has a running transition or completed transition for the property, and there is not a matching transition-property value, then implementations must cancel the running transition or remove the completed transition from the set of completed transitions.
4. If the element has a running transition for the property, there is a matching transition-property value, and the end value of the running transition is not equal to the value of the property in the after-change style, then:
   1. If the current value of the property in the running transition is equal to the value of the property in the after-change style, or if these two values are not transitionable, then implementations must cancel the running transition.
   2. Otherwise, if the combined duration is less than or equal to 0s, or if the current value of the property in the running transition is not transitionable with the value of the property in the after-change style, then implementations must cancel the running transition.
   3. Otherwise, if the reversing-adjusted start value of the running transition is the same as the value of the property in the after-change style (see the section on reversing of transitions for why these case exists), implementations must cancel the running transition and start a new transition whose:
      • reversing-adjusted start value is the end value of the running transition (Note: This represents the logical start state of the transition, and allows some calculations to ignore that the transition started before that state was reached, which in turn allows repeated reversals of the same transition to work correctly),
      • reversing shortening factor is the absolute value, clamped to the range [0, 1], of the sum of:
        1. the output of the timing function of the old transition at the time of the style change event, times the reversing shortening factor of the old transition
        2. 1 minus the reversing shortening factor of the old transition.
        Note: This represents the portion of the space between the reversing-adjusted start value and the end value that the old transition has traversed (in amounts of the value, not time), except with the absolute value and clamping to handle timing functions that have y1 or y2 outside the range [0, 1].
      • start time is the time of the style change event plus:
        1. if the matching transition delay is nonnegative, the matching transition delay, or
        2. if the matching transition delay is negative, the product of the new transition’s reversing shortening factor and the matching transition delay,
      • end time is the start time plus the product of the matching transition duration and the new transition’s reversing shortening factor,
      • start value is the current value of the property in the running transition,
      • end value is the value of the property in the after-change style,
   4. Otherwise, implementations must cancel the running transition and start a new transition whose:
      • start time is the time of the style change event plus the matching transition delay,
      • end time is the start time plus the matching transition duration,
      • start value is the current value of the property in the running transition,
      • end value is the value of the property in the after-change style,
      • reversing-adjusted start value is the same as the start value, and
      • reversing shortening factor is 1.

li {
  transition: background-color linear 1s;
  background: blue;
}
li:hover {
  background-color: green;
  transition-duration: 2s; /* applies to the transition *to* the :hover state */
}

Note that once the transition of a property has started (including being in its delay phase), it continues 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 stops (and the property immediately changes to its final value).

Note that above rules mean that transitions do not start when the computed value of a property changes as a result of declarative animation (as opposed to scripted animation). This happens because the before-change style includes up-to-date style for declarative animations.

3.1. Faster reversing of interrupted transitions

4. Application of transitions

When a property on an element is undergoing a transition (that is, when or after the transition has started and before the end time of the transition) the transition adds a style called the current value to the CSS cascade at the level defined for CSS Transitions in [CSS3CASCADE].

Note that this means that computed values resulting from CSS transitions can inherit to descendants just like any other computed values. In the normal case, this means that a transition of an inherited property applies to descendant elements just as an author would expect.

Implementations must add this value to the cascade if and only if that property is not currently undergoing a CSS Animation ([CSS3-ANIMATIONS]) on the same element.

Note that this behavior of transitions not applying to the cascade when an animation on the same element and property is running does not affect whether the transition has started or ended. APIs that expose whether transitions are running (such as transition events) still report that a transition is running.

If the current time is at or before the start time of the transition (that is, during the delay phase of the transition), the current value is a specified style that will compute to the start value of the transition.

If the current time is after the start time of the transition (that is, during the duration phase of the transition), the current value is a specified style that will compute to the result of interpolating the property using the following values:

• V_a: start value of the transition

• V_b: end value of the transition

• p: the output of the timing function for input (current time - start time) / (end time - start time)

The specific interpolation procedure to be used is defined by the property’s animation type.

5. Completion of transitions

Running transitions complete at a time that is equal to or after their end time, but prior to the first style change event whose time is equal to or after their end time. When a transition completes, implementations must move all transitions that complete at that time from the set of running transitions to the set of completed transitions and then fire the events for those completions. (Note that doing otherwise, that is, firing some of the events before doing all of the moving from running transitions to completed transitions, could allow a style change event to happen without the necessary transitions completing, since firing the event could cause a style change event, if an event handler requests up-to-date computed style or layout data.)

6. Transition Events

The creation, beginning, completion, and cancellation of CSS transitions generate corresponding DOM Events. An event is dispatched to the element for each property that undergoes a transition on that element. This allows a content developer to perform actions that synchronize with changes to transitions.

Each event provides the name of the property the transition is associated with as well as the duration of the transition.

6.1. Interface TransitionEvent

The TransitionEvent interface provides specific contextual information associated with transitions.

6.1.1. IDL Definition

[Exposed=Window]
interface TransitionEvent : Event {
  constructor(CSSOMString type, optional TransitionEventInit transitionEventInitDict = {});
  readonly attribute CSSOMString propertyName;
  readonly attribute double elapsedTime;
  readonly attribute CSSOMString pseudoElement;
};

dictionary TransitionEventInit : EventInit {
  CSSOMString propertyName = "";
  double elapsedTime = 0.0;
  CSSOMString pseudoElement = "";
};

6.1.2. Attributes

propertyName, of type CSSOMString, readonly

The name of the CSS property associated with the transition.

Note: This is always the name of a longhand property. See transition-property for how specifying shorthand properties causes transitions on longhands.

elapsedTime, of type double, readonly

The amount of time the transition has been running, in seconds, when this event fired not including any time spent in the delay phase. The calculation for of this member is defined along with each event type.

pseudoElement, of type CSSOMString, readonly

The name (beginning with two colons) of the CSS pseudo-element on which the transition occurred (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, transitionEventInitDict) is an event constructor.

6.2. Types of TransitionEvent

The different types of transition events that can occur are:

transitionrun

The transitionrun event occurs when a transition is created (i.e., when it is added to the set of running transitions).

A negative transition-delay will cause the event to fire with an elapsedTime equal to the absolute value of the delay capped to the transition-duration of the animation. That is, the elapsed time is equal to min(max(-transition-delay, 0), transition-duration).

• Bubbles: Yes
• Cancelable: No
• Context Info: propertyName, elapsedTime, pseudoElement

transitionstart

The transitionstart event occurs when a transition’s delay phase ends.

The value of elapsedTime for transitionstart events is the same as the value used for transitionrun events.

• Bubbles: Yes
• Cancelable: No
• Context Info: propertyName, elapsedTime, pseudoElement

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.

The value of elapsedTime for this event is equal to the value of transition-duration.

• Bubbles: Yes
• Cancelable: No
• Context Info: propertyName, elapsedTime, pseudoElement

transitioncancel

The transitioncancel event occurs when a transition is canceled.

The elapsedTime for transitioncancel events is the number of seconds from the end of the transition’s delay to the moment when the transition was canceled. If the transition had a negative transition-delay, the beginning of the transition is the moment equal to the absolute value of transition-delay seconds prior to when the transition was actually triggered. Alternatively, if the transition had a positive transition-delay and the event is fired before the transition’s delay has expired, the elapsedTime will be zero.

• Bubbles: Yes
• Cancelable: No
• Context Info: propertyName, elapsedTime, pseudoElement

6.3. Event handlers on elements, Document objects, and Window objects

The following are the event handlers (and their corresponding event handler event types) that must be supported by all HTML elements, as both event handler content attributes and event handler IDL attributes; and that must be supported by all Document and Window objects, as event handler IDL attributes:

7. DOM Interfaces

This specification extends the GlobalEventHandlers interface mixin from HTML to add event handler IDL attributes for transition events as defined in § 6.3 Event handlers on elements, Document objects, and Window objects.

7.1. IDL Definition

partial interface mixin GlobalEventHandlers {
  attribute EventHandler ontransitionrun;
  attribute EventHandler ontransitionstart;
  attribute EventHandler ontransitionend;
  attribute EventHandler ontransitioncancel;
};

8. Security Considerations

This section is not normative.

The security implications of this specification are limited because it doesn’t allow Web content to do things that it could not do before. Rather, it allows things that could previously be done with script to be done declaratively, and it ways that implementations can optimize (for frame rate and CPU usage).

One of the major categories of optimizations that implementations can make is implementing animation of certain high-value properties (such as transform and opacity) run on a browser’s compositor thread or process without updating style or layout on the main Web content thread unless up-to-date style data are needed. This optimization often requires allocations of graphics memory to display the contents of the element being animated. Implementations should take care to ensure that Web content cannot trigger unsafe out-of-memory handling by using large numbers of animations or animations on elements covering large areas (where large may be defined in terms of pre-transform or post-transform size).

9. Privacy Considerations

This section is not normative.

As for security, the privacy considerations of this specification are limited because it does not allow Web content to do things that it could not do before.

This specification may provide additional mechanisms that help to determine characteristics of the user’s hardware or software. However, ability to determine performance characteristics of the user’s hardware or software is common to many Web technologies, and this specification does not introduce new capabilities.

As described in § 10 Accessibility Considerations, implementations may provide mitigations to help users with disabilities. These mitigations are likely to be detectable by Web content, which means that users who would benefit from these mitigations may face a tradeoff between keeping their disability private from the Web content or benefiting from the mitigation.

10. Accessibility Considerations

This section is not normative.

10.1. Motion

This specification provides declarative mechanisms for animations that previously needed to be done using script. Providing a declarative mechanism has multiple effects: it makes such animations easier to make and thus likely to be more common, but it also makes it easier for user agents to modify those animations if such modifications are needed to meet a user’s accessibility needs.

Thus, users who are sensitive to movement, or who require additional time to read or understand content, may benefit from user agent features that allow animations to be disabled or slowed down. (But see § 9 Privacy Considerations for information on the privacy implications of such mitigations.)

User agent implementors should be aware that Web content may depend on the firing of transition events, so implementations of such mitigations may wish to fire transition events even if the transitions were not run as continuous animations. However, it is probably poor practice for Web content to depend on such events to function correctly.

10.2. Cascade

The CSS cascade is a general mechanism in CSS that allows user needs to interact with author styles. This specification interacts with the cascade, but since it only allows animation between values that result from the existing cascade rules, it does not interfere with the user’s ability to force CSS properties to have particular values.

The cascade also allows users to disable transitions entirely by overriding the transition properties.

11. Changes

11.1. Changes since Working Draft of 11 October 2018

The following are the substantive changes made since the Working Draft dated 11 October 2018:

• (Not up to date)

For more details on these changes, see the version control change log.

11.2. Earlier changes

For changes in earlier working drafts:

1. See the changes section in the 11 October 2018 Working Draft.

2. See the changes section in the 30 November 2017 Working Draft.

3. See the changes section in the 19 November 2013 Working Draft.

4. See the the ChangeLog for changes in previous working drafts.

5. For more details on these changes, see the version control change logs, which are split in many parts because of file renaming:

   • change log since 2017 October 12,

   • change log from 2015 January 26 to 2017 October 12,

   • change log from 2013 March 28 to 2015 January 26,

   • change log before 2013 March 28.

12. Acknowledgments

Thanks especially to the feedback from Tab Atkins, Carine Bournez, Aryeh Gregor, Vincent Hardy, Anne van Kesteren, Cameron McCormack, Alex Mogilevsky, Jasper St. Pierre, Estelle Weyl, and all the rest of the www-style community.