Status: ED Work Status: Refining ED: http://dev.w3.org/csswg/css-transitions/ Shortname: css-transitions Group: csswg Level: 1 TR: http://www.w3.org/TR/css3-transitions/ Previous version: http://www.w3.org/TR/2013/WD-css3-transitions-20131119/ ED: http://dev.w3.org/csswg/css-transitions/ Editor: L. David Baron, Mozilla, http://dbaron.org/ Editor: Dean Jackson, Apple Inc, dino@apple.com Editor: David Hyatt, Apple Inc, hyatt@apple.com Editor: Chris Marrin, Apple Inc, cmarrin@apple.com Issue Tracking: Bugzilla bugs for this level https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&product=CSS&component=Transitions&resolution=---&status_whiteboard=defer%20to%20level%202&status_whiteboard_type=notregexp Issue Tracking: Bugzilla bugs for all levels https://www.w3.org/Bugs/Public/buglist.cgi?query_format=advanced&product=CSS&component=Transitions&resolution=--- 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 Terms: floatThe following behaviors are at risk:
spec:css21; 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-color-3;
type:property;
text:color
text:opacity
type:value
text:green
text:blue
text:transparent
spec:css-values-3; type:type; text:
url: http://dev.w3.org/csswg/css-backgrounds-3/#shadow-inset; type: value; for: shadow; text: inset;
url: http://www.w3.org/TR/CSS21/visufx.html#propdef-visibility; type: value; for: visibility; text: visible;
urlPrefix: http://dev.w3.org/csswg/css-color-3/; type: value;
text: transparent
text: blue
text: green
url: http://w3c.github.io/dom/#constructing-events; type: dfn; text: event constructor;
url: https://html.spec.whatwg.org/multipage/infrastructure.html#concept-event-dispatch; type: dfn; text: dispatch;
This section is not normative.
This document introduces new CSS features to enable implicit transitions, which describe how CSS properties can be made to change smoothly from one value to another over a given duration.
Normally when the value of a CSS property changes, the rendered result is instantly updated, with the affected elements immediately changing from the old property value to the new property value. This section describes a way to specify transitions using new CSS properties. These properties are used to animate smoothly from the old state to the new state over time.
For example, suppose that transitions of one second have been defined on the 'left' and 'background-color' properties. The following diagram illustrates the effect of updating those properties on an element, in this case moving it to the right and changing the background from red to blue. This assumes other transition parameters still have their default values.
Transitions 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 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.
Only animatable CSS properties can be transitioned. See the table at the end of this document for a list of properties that are animatable.
The transition for a property is defined using a number of new properties. For example:
div {
transition-property: opacity;
transition-duration: 2s;
}
The above example defines a transition on the 'opacity' property that, when a new value is assigned to it, will cause a smooth change between the old value and the new value over a period of two seconds.
Each of the transition properties accepts a comma-separated list, allowing multiple transitions to be defined, each acting on a different property. In this case, the individual transitions take their parameters from the same index in all the lists. For example:
div {
transition-property: opacity, left;
transition-duration: 2s, 4s;
}
This will cause the 'opacity' property to transition over a period of two seconds and the left property to transition over a period of four seconds.
In the case where the lists of values in transition properties do not have the same length, the length of the 'transition-property' list determines the number of items in each list examined when starting transitions. The lists are matched up from the first value: excess values at the end are not used. If one of the other properties doesn't have enough comma-separated values to match the number of values of 'transition-property', the UA must calculate its used value by repeating the list of values until there are enough. This truncation or repetition does not affect the computed value. Note: This is analogous to the behavior of the 'background-*' properties, with 'background-image' analogous to 'transition-property'.
div {
transition-property: opacity, left, top, width;
transition-duration: 2s, 1s;
}
The above example defines a transition on the 'opacity' property of 2 seconds duration, a
transition on the 'left' property of 1
second duration, a transition on the 'top' property of 2 seconds duration and a
transition on the 'width' property of 1
second duration.
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: ''transition-property/none'' | <>#
Initial: ''transition-property/all''
Applies to: all elements, ::before and ::after pseudo elements
Inherited: no
Animatable: no
Percentages: N/A
Media: visual
Computed value: Same as specified value.
Canonical order: per grammar
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 <
For the keyword ''transition-property/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 ''transition-property/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 ''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: <
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. 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
Name: transition-timing-function
Value: <>#
Initial: ''transition-timing-function/ease''
Applies to: all elements, ::before and ::after pseudo elements
Inherited: no
Animatable: no
Percentages: N/A
Media: interactive
Computed value: Same as specified value.
Canonical order: per grammar
The timing functions have the following definitions.
The 'transition-delay' property defines when the transition will start. It allows a transition to begin execution some some period of time from when it is applied. A 'transition-delay' value of ''0s'' means the transition will execute as soon as the property is changed. Otherwise, the value specifies an offset from the moment the property is changed, and the transition will delay execution by that offset.
If the value for 'transition-delay' is a negative time offset then the transition will execute the moment the property is changed, but will appear to have begun execution at the specified offset. That is, the transition will appear to begin part-way through its play cycle. In the case where a transition has implied starting values and a negative 'transition-delay', the starting values are taken from the moment the property is changed.
Name: transition-delay
Value: <
The 'transition' shorthand property combines the four properties described above into a single property.
Name: transition
Value: <>#
Initial: see individual properties
Applies to: all elements, ::before and ::after pseudo elements
Inherited: no
Animatable: no
Percentages: N/A
Media: interactive
Computed value: see individual properties
Canonical order: per grammar
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 <
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 remove transitions on it from the running transitions and the completed transitions.
This set of completed transitions needs to be maintained in order to prevent transitions from repeating themselves in certain cases, i.e., to maintain the invariant that this specification tries to maintain that unrelated style changes do not trigger transitions.
An example where maintaining the set of completed transitions is necessary would be a transition on an inherited property, where the parent specifies a transition of that property for a longer duration (say, ''transition: 4s text-indent'') and a child element that inherits the parent's value specifies a transition of the same property for a shorter duration (say, ''transition: 1s text-indent''). Without the maintenance of this set of completed transitions, implementations could start additional transitions on the child after the initial 1 second transition on the child completes.
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 even 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 excluding any styles from CSS Transitions in the computation, and inheriting from the after-change style of the parent.
Note that this definition of the after-change style means that a single change can start a transition on the same property on both an ancestor element and its descendant element. This can happen when a property change is inherited from one element with 'transition-*' properties that say to animate the changing property to another element with 'transition-*' properties that also say to animate the changing property.
When this happens, both transitions will run, and the transition on the descendant will override the transition on the ancestor because of the normal CSS cascading and inheritance rules ([[CSS3CASCADE]]).
If the transition on the descendant completes before the transition on the ancestor, the descendant will then resume inheriting the (still transitioning) value from its parent. This effect is likely not a desirable effect, but it is essentially doing what the author asked for.
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. For each element and property, the implementation must act as follows:
Note that the above rules mean that when the computed value of an animatable property changes, the transitions that start are based on the values of the 'transition-property', 'transition-duration', 'transition-timing-function', and 'transition-delay' properties at the time the animatable property would first have its new computed value. This means that when one of these 'transition-*' properties changes at the same time as a property whose change might transition, it is the new values of the 'transition-*' properties that control the transition.
This provides a way for authors to specify different values of the 'transition-*' properties for the “forward” and “reverse” transitions, when the transitions are between two states (but see below for special reversing behavior when an incomplete transition is interrupted). Authors can specify the value of 'transition-duration', 'transition-timing-function', or 'transition-delay' in the same rule where they specify the value that triggers the transition, or can change these properties at the same time as they change the property that triggers the transition. Since it's the new values of these 'transition-*' properties that affect the transition, these values will be used for the transitions to the associated transitioning values. For example:
li {
transition: background-color linear 1s;
background: blue;
}
li:hover {
background-color: green;
transition-duration: 2s; /* applies to the transition *to* the :hover state */
}
When a list item with these style rules enters the :hover state, the computed 'transition-duration' at the time that 'background-color' would have its new value (''green'') is ''2s'', so the transition from ''blue'' to ''green'' takes 2 seconds. However, when the list item leaves the :hover state, the transition from ''green'' to ''blue'' takes 1 second.
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.
Many common transitions effects involve transitions between two states, such as the transition that occurs when the mouse pointer moves over a user interface element, and then later moves out of that element. With these effects, it is common for a running transition to be interrupted before it completes, and the property reset to the starting value of that transition. An example is a hover effect on an element, where a transition starts when the pointer enters the element, and then the pointer 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 because the second transition takes the full specified time to move a shortened distance. Instead, this specification makes second transition shorter.
The mechanism the above rules use to cause this involves the reversing shortening factor and the reversing-adjusted start value. In particular, the reversing behavior is present whenever the reversing shortening factor is less than 1.
Note that these rules do not fully address the problem for transition patterns that involve more than two states.
Note that these rules lead to the entire timing function of the new transition being used, rather than jumping into the middle of a timing function, which can create a jarring effect.
This was one of several possibilities that was considered by the working group. See the reversing demo demonstrating a number of them, leading to a working group resolution made on 2013-06-07 and edits made on 2013-11-11.
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 detect 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 start value of the transition as Vstart, using the end value of the transition as Vend, and using (current time - start time) / (end time - start time) as the input to the timing function.
Running transitions complete at a time that equal to or after their end time, but prior to 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.)
The completion of a CSS Transition generates a corresponding DOM Event. 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 the completion of a transition.
Each event provides the name of the property the transition is associated with as well as the duration of the transition.
The {{TransitionEvent}} interface provides specific contextual information associated with transitions.
[Constructor(DOMString type, optional TransitionEventInit transitionEventInitDict)]
interface TransitionEvent : Event {
readonly attribute DOMString propertyName;
readonly attribute float elapsedTime;
readonly attribute DOMString pseudoElement;
};
dictionary TransitionEventInit : EventInit {
DOMString propertyName = "";
float elapsedTime = 0.0;
DOMString pseudoElement = "";
};
propertyName of type DOMString, readonly
elapsedTime of type float, readonly
transition-delay.
pseudoElement of type DOMString, readonly
TransitionEvent(type, transitionEventInitDict)
is an event constructor.
There is one type of transition event available.
Some property types can be interpolated, which means they can animate smoothly from one value to another. Other property types cannot, and thus animate only in a single step from one value to the other.
When interpolating between two values, Vstart and Vend, 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 Vres = (1 - p) ⋅ Vstart + p ⋅ Vend.
However, if this value (Vres) 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.
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.
When interpolating between two values that cannot be interpolated, Vstart and Vend, interpolation is done using the output p of the timing function. If p is less than 0.5, then the result of the interpolation is Vstart; if p is greater than or equal to 0.5, then the result of the interpolation is Vend.
This is a recent change to which implementations have not yet updated. (Prior to the change CSS Transitions and CSS Animations did not run on such changes.) It's possible that it won't be compatible with existing Web content. If that is the case, the problem may be mitigated by restricting this behavior only to CSS Animations (and not to CSS Transitions), and/or restricting it to step timing functions.
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. The animated value is interpolated from the from and to values 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.
Otherwise, since the from and to values cannot be interpolated, the animation is done in a single step.
The 'transition-*' properties defined in this specification do not undergo transitions.
For properties that exist at the time this specification was developed, this specification defines 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.
It no longer makes sense for this line to be called "Animatable". It should probably be renamed to "Interpolation", and the "no" value renamed to "discrete" or "in steps".
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]]).
| Property Name | Type |
|---|---|
| 'background-color' | as color |
| 'background-position' | as repeatable list of simple list of length, percentage, or calc |
| 'border-bottom-color' | as color |
| 'border-bottom-width' | as length |
| 'border-left-color' | as color |
| 'border-left-width' | as length |
| 'border-right-color' | as color |
| 'border-right-width' | as length |
| 'border-spacing' | as simple list of length |
| 'border-top-color' | as color |
| 'border-top-width' | as length |
| 'bottom' | as length, percentage, or calc |
| 'clip' | as rectangle |
| 'color' | as color |
| 'font-size' | as length |
| 'font-weight!!property' | as font weight |
| 'height' | as length, percentage, or calc |
| 'left' | as length, percentage, or calc |
| 'letter-spacing' | as length |
| 'line-height' | as either number or length |
| 'margin-bottom' | as length |
| 'margin-left' | as length |
| 'margin-right' | as length |
| 'margin-top' | as length |
| 'max-height' | as length, percentage, or calc |
| 'max-width' | as length, percentage, or calc |
| 'min-height' | as length, percentage, or calc |
| 'min-width' | as length, percentage, or calc |
| 'opacity' | as number |
| 'outline-color' | as color |
| 'outline-width' | as length |
| 'padding-bottom' | as length |
| 'padding-left' | as length |
| 'padding-right' | as length |
| 'padding-top' | as length |
| 'right' | as length, percentage, or calc |
| 'text-indent' | as length, percentage, or calc |
| 'text-shadow' | as shadow list |
| 'top' | as length, percentage, or calc |
| 'vertical-align' | as length |
| 'visibility' | as visibility |
| 'width' | as length, percentage, or calc |
| 'word-spacing' | as length |
| 'z-index' | as integer |
All properties defined as animatable in the SVG specification, provided they are one of the property types listed above.
The following are the substantive changes made since the Working Draft dated 19 November 2013:
For more details on these changes, see the version control change log since 2015 January 26 and the change log from 2013 March 28 to 2015 January 26.
For changes in earlier working drafts:
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.