Title: CSS Transitions Level 1
Status: ED
Work Status: Refining
Shortname: css-transitions
Group: csswg
Level: 1
TR: https://www.w3.org/TR/css-transitions-1/
Previous version: https://www.w3.org/TR/2018/WD-css-transitions-1-20181011/
Previous version: https://www.w3.org/TR/2017/WD-css-transitions-1-20171130/
Previous version: https://www.w3.org/TR/2013/WD-css3-transitions-20131119/
ED: https://drafts.csswg.org/css-transitions/
Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
Editor: Dean Jackson, Apple Inc https://www.apple.com/, dino@apple.com, w3cid 42080
Editor: Brian Birtles, Mozilla https://www.mozilla.org/, bbirtles@mozilla.com, w3cid 43194
Former Editor: David Hyatt, Apple Inc https://www.apple.com/, hyatt@apple.com, w3cid 34140
Former Editor: Chris Marrin, Apple Inc https://www.apple.com/, cmarrin@apple.com
Issue Tracking: Bugzilla bugs for all levels https://www.w3.org/Bugs/Public/buglist.cgi?product=CSS&component=Transitions&resolution=---
Test Suite: https://wpt.fyi/results/css/css-transitions
Abstract: CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.
Status Text: This document is expected to be relatively close to last call.  While some issues raised have yet to be addressed, new features are extremely unlikely to be considered for this level.
Ignored Vars: x1, x2, y1, y2
Link Defaults: css-transforms (property) transform

spec:css22; type:property;
    text:top
    text:right
    text:bottom
    text:left
    text:margin-top
    text:margin-right
    text:margin-bottom
    text:margin-left
    text:padding-top
    text:padding-right
    text:padding-bottom
    text:padding-left
    text:border-top-color
    text:border-right-color
    text:border-bottom-color
    text:border-left-color
    text:border-top-width
    text:border-right-width
    text:border-bottom-width
    text:border-left-width
    text:background-color
    text:background-position
    text:border-spacing
    text:width
    text:height
    text:min-width
    text:min-height
    text:max-width
    text:max-height
    text:clip
    text:letter-spacing
    text:line-height
    text:outline-color
    text:outline-width
    text:text-indent
    text:font-size
    text:font-weight
    text:vertical-align
    text:visibility
    text:word-spacing
    text:z-index
spec:css-backgrounds-3; type:property;
    text:background-image
    text:background-origin
spec:css-color-4;
    type:property;
        text:color
        text:opacity
    type:value
        text:green
        text:blue
        text:transparent
spec:css-values-3; type:type; text:

url: https://www.w3.org/TR/css3-background/#shadow-inset; type: value; for: shadow; text: inset;
url: https://www.w3.org/TR/css3-background/#box-shadow-none; type: value; for: shadow; text: none;
url: https://www.w3.org/TR/CSS21/visufx.html#propdef-visibility; type: value; for: visibility; text: visible;
urlPrefix: https://www.w3.org/TR/css3-color/; type: value;
    text: transparent
    text: blue
    text: green
url: http://w3c.github.io/dom/#constructing-events; type: dfn; text: event constructor;
urlPrefix: https://html.spec.whatwg.org/multipage/webappapis.html; type: dfn; spec: html
    text: event handlers
    text: event handler event type
    text: event handler content attributes
    text: event handler IDL attributes
urlPrefix: https://html.spec.whatwg.org/multipage/infrastructure.html; type: dfn; spec: html
    text: HTML elements
    text: dispatch; url: concept-event-dispatch
url: https://html.spec.whatwg.org/#document; type: interface; text: Document; spec: html

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 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]]).

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

        Name: transition-property
        Value: none | <>#
        Initial: all
        Applies to: all elements
        Inherited: no
        Percentages: N/A
        Computed value: the keyword ''transition-property/none'' else a list of identifiers
        Animation type: not animatable
      

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 <> production in <> also excludes the keyword ''transition-property/none'', in addition to the keywords always excluded from <>. This means that ''transition-property/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 ''transition-property/all'', or if one of the identifiers listed is a shorthand property, implementations must start transitions for all its longhand sub-properties (or, for ''transition-property/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 ''transition-property/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 ''transition-property/all'' value and 'all' shorthand property work in similar ways, so the ''transition-property/all'' value is just like a shorthand that covers all properties.

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

        Name: transition-duration
        Value: <>#
        Initial: ''0s''
        Applies to: all elements
        Inherited: no
        Percentages: N/A
        Computed value: list, each item a duration
        Animation type: not animatable
      

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.

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 [[#application]]).

        Name: transition-timing-function
        Value: <>#
        Initial: ''transition-timing-function/ease''
        Applies to: all elements
        Inherited: no
        Percentages: N/A
        Computed value: as specified
        Animation type: not animatable
      

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.

        Name: transition-delay
        Value: <>#
        Initial: ''0s''
        Applies to: all elements
        Inherited: no
        Percentages: N/A
        Computed value: list, each item a duration
        Animation type: not animatable
      

The 'transition' shorthand property combines the four properties described above into a single property.

        Name: transition
        Value: <>#
        Applies to: all elements
        Inherited: no
        Percentages: N/A
        Animation type: not animatable
      

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 <> in the shorthand, and any of the transitions has ''transition-property/none'' as the <>, then the declaration is invalid.

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 [[#reversing]].

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 [[#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.

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.

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)

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.)

  [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 = "";
  };

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

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 {{TransitionEvent/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 {{TransitionEvent/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 {{TransitionEvent/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 {{TransitionEvent/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 {{AnimationEvent/elapsedTime}} will be zero.

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

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

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.