Title: CSS Transitions Level 2
Status: ED
Work Status: Exploring
Shortname: css-transitions-2
Level: 2
Group: csswg
ED: https://drafts.csswg.org/css-transitions-2/
Editor: L. David Baron, Mozilla https://www.mozilla.org/, https://dbaron.org/, w3cid 15393
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.
Ignored Terms: translate, rotate, scale

urlPrefix: https://w3c.github.io/web-animations/; type: interface; spec: web-animations
    text: Animation
urlPrefix: https://w3c.github.io/web-animations/; type: dfn; spec: web-animations
    text: active duration
    text: active time
    text: animation
    text: animation class
    text: animation playback rate
    text: current iteration
    text: current time
    text: fill mode
    text: global animation list
    text: idle play state
    text: iteration duration
    text: iteration start
    text: pending play state
    text: sampling
    text: start delay
    text: target effect
    text: target effect end
    text: unresolved

spec:css-transitions-1; type:value; text:all

Delta specification

This is a delta specification, meaning that it currently contains only the differences from CSS Transitions Level 1 [[!CSS3-TRANSITIONS]]. Once the Level 1 specification is closer to complete, it will be merged with the additions here into a complete level 2 specification.

# Starting of transitions # {#starting} Associated with each top-level browsing context is a current transition generation that is incremented on each style change event. Each time a new transition is generated, the current value of the (already incremented) current transition generation is stored as the transition's transition generation. ## The transition property name ## {#transition-property-name} Although the 'transition-property' may specify shorthand properties and the ''all'' keyword, individual transitions are generated for each longhand sub-property that is animatable. The expanded transition property name of a transition is the name of the longhand sub-property for which the transition was generated (e.g. 'border-left-width'). ## Owning element ## {#owning-element-section} The owning element of a transition refers to the element or pseudo-element to which the 'transition-property' property was applied that generated the animation. If a transition was generated directly by script (e.g. using the {{CSSTransition}} constructor) then it has no owning element. If a transition generated using the markup defined in this specification is later disassociated from that markup because it is cancelled or replaced by a newer transition, the animation is disassociated from its owning element (that is, it has no owning element from that point forwards). Issue: Define the above more precisely once we rewrite firing of transitions in terms of Web Animations concepts (specifically when we spell out when we cancel an animation). ## Animation composite order ## {#animation-composite-order} Animations generated from the markup and interfaces (e.g. the {{CSSTransition}} constructor) defined in this specification have an animation class of ‘CSS Transition’. CSS Transitions have an earlier composite order that CSS Animations and animations without a specific animation class. Within the set of CSS Transitions, two animations A and B are sorted in composite order (first to last) as follows: 1. If neither A nor B has an owning element, sort based on their relative position in the global animation list. 1. Otherwise, if only one of A or B has an owning element, let the animation with an owning element sort first. 1. Otherwise, if the owning element of A and B differs, sort A and B by tree order of their corresponding owning elements. With regard to pseudo-elements, the sort order is as follows: * element * ::before * ::after * element children 1. Otherwise, if A and B have different transition generation values, sort by their corresponding transition generation in ascending order. 1. Otherwise, sort A and B in ascending order by the Unicode codepoints that make up the expanded transition property name of each transition (i.e. without attempting case conversion and such that ‘-moz-column-width’ sorts before ‘column-width’). Transitions generated using the markup defined in this specification are not added to the global animation list when they are created. Instead, these animations are appended to the global animation list at the first moment when they transition out of the idle play state after being disassociated from their owning element. Transitions that have been disassociated from their owning element but are still idle do not have a defined composite order. Note, this behavior relies on the fact that disassociating a transition from its owning element always causes it to enter (or remain) in the idle play state. Transitions created using the {{CSSTransition}} constructor are appended to the global animation list at the moment they are constructed. # Transition Events # {#transition-events} ## Event dispatch ## {#event-dispatch} Note, this is a more general description of event dispatch than that of CSS Transitions Level 1 [[CSS3-TRANSITIONS]] since it must account for the possibility of animations being seeked or reversed using the Web Animations API [[WEB-ANIMATIONS]]. Furthermore, it is possible using the Web Animations API to substitute the transition effect with an entirely different effect with properties not normally used with transitions (e.g. an effect that repeats multiple times) and hence this section provides a generic definition that accounts for the full complexity of the Web Animations model. To avoid firing redundant events, the set of events to dispatch is based on comparing the phase of the transition in the previous sample (animation frame) to its current state. The transition phase of a transition is initially ‘idle’ and is updated on each sample according to the first matching condition from below:
If the transition has no target effect,
The transition phase is set according to the first matching condition from below:
If the transition has an unresolved current time,
The transition phase is ‘idle’.
If the transition has a current time < 0,
The transition phase is ‘before’.
Otherwise,
The transition phase is ‘after’.
If the transition is pending and its phase was previously ‘idle’ or ‘pending’,
The transition phase is ‘pending’.
Otherwise,
The transition phase is the [[web-animations#animation-effect-phases-and-states|phase]] of its target effect.
For calculating the {{TransitionEvent/elapsedTime}} of each event, the following definitions are used: * interval start = max(min(-start delay, active duration), 0) * interval end = max(min(target effect end - start delay, active duration), 0) In the above formulae, references to the start delay, active duration, current iteration, iteration start, and iteration duration of a transition should be understood to refer to the corresponding properties of the transition's target effect. Each time an animation is sampled, the events to dispatch are determined by comparing the transition phase before and after the sample as follows:
Change Events dispatched Elapsed time (ms)
idle → pending or before {{transitionrun}} interval start
idle → active ٭ {{transitionrun}} interval start
{{transitionstart}}
idle → after ٭ {{transitionrun}} interval start
{{transitionstart}}
{{transitionend}} interval end
pending or before → active {{transitionstart}} interval start
pending or before → after ٭ {{transitionstart}} interval start
{{transitionend}} interval end
active → after {{transitionend}} interval end
active → before {{transitionend}} interval start
after → active {{transitionstart}} interval end
after → before ٭ {{transitionstart}} interval end
{{transitionend}} interval start
not idle and not after → idle {{transitioncancel}} The active time of the animation at the moment it was cancelled calculated using a fill mode of both.

٭ Where multiple events are listed for a state change, all events are dispatched in the order listed and in immediate succession.

Since the elapsed time defined in the table and procedure above is expressed in milliseconds, it must be divided by 1,000 to produce a value in seconds before being assigned to the {{TransitionEvent/elapsedTime}} member of the {{TransitionEvent}}.
The above state transition chart ensures that, with the exception of transitions that are paused or have an infinite running time, the following invariants hold: * For every {{transitionrun}} event there will be a exactly one {{transitionend}} or {{transitioncancel}} and never both. * For every {{transitionstart}} event there will be a exactly one {{transitionend}} or {{transitioncancel}} and never both. * Every {{transitionend}} event is preceded by a corresponding {{transitionstart}} event. The typical sequences of events, then, are as follows: * Regular playback: {{transitionrun}}, {{transitionstart}}, {{transitionend}}. * Interrupted playback: {{transitionrun}}, {{transitionstart}}, {{transitioncancel}}. * Interrupted playback during delay or pending phase: {{transitionrun}}, {{transitioncancel}}. * Reversed playback after completion: {{transitionrun}}, {{transitionstart}}, {{transitionend}}, {{transitionstart}}, {{transitionend}}.
# DOM Interfaces # {#interface-dom} ## The CSSTransition interface ## {#the-CSSTransition-interface} 
[Exposed=Window]
interface CSSTransition : Animation {
  readonly attribute CSSOMString transitionProperty;
};
: transitionProperty :: The expanded transition property name of this transition.
This interface needs a constructor. Perhaps something like the following, 
[Constructor (Animatable? target,
              CSSOMString transitionProperty,
              any transitionValue,
              optional (unrestricted double or KeyframeEffectOptions) options),
 Constructor (Animatable? target,
              CSSOMString transitionProperty,
              any transitionValue,
              (unrestricted double or KeyframeEffectOptions) options,
              AnimationTimeline? timeline)]
partial interface CSSTransition { };
Using any as the type for transitionValue would allow the following usage: 
  var transition = new CSSTransition(elem, 'opacity', 1, 2000);
  transition.play();
The above example, however, is not equivalent to triggering a transition. It has the following differences: * It does not update the specified style. * It would not cancel any existing transition on the given property unless we add special wording to that effect. * It would exhibit different composite order to a regular transition (this should be easily fixed, however). * It has been suggested that script-generated animations should not apply to the transition level of the cascade. It is unclear how much we should extend the {{CSSTransition}} constructor to overcome the above limitations so that it can be used to generate transitions programmatically, or whether we should introduce a separate helper method such as has been suggested elsewhere.
## Requirements on pending style changes ## {#requirements-on-pending-style-changes} Various operations may affect the computed values of properties on elements. User agents may, as an optimization, defer recomputing these values until it becomes necessary. However, all operations included in programming interface defined in this specification, as well as those operations defined in Web Animations [[!WEB-ANIMATIONS]] that may return objects defined by this specification, must produce a result consistent with having fully processed any such pending changes to computed values.
As an example, in the following code fragment, when the specified value of elem's 'opacity' property is updated, a user agent may defer recalculating the computed value of the 'animation' property. The first time this occurs, calling {{Window/getComputedStyle(elt)}} and subsequently accessing the opacity property of the result will cause the user agent to recompute the value of opacity. After the 'opacity' property is updated a second time, the {{Animatable/getAnimations()}} method is called on elem. This method is specified by Web Animations and can return {{CSSTransition}} objects as defined in this specification. Hence, as result of the requirements in this section, the user agent must apply any pending style changes thus generating a new {{CSSTransition}} for the 'opacity' property before returning its result. 
elem.style.transition = 'opacity 100s';
elem.style.opacity = '0';
window.getComputedStyle(elem).opacity; // 0
elem.style.opacity = '1';
elem.getAnimations()[0].transitionProperty // 'opacity'

Issues commonly raised as issues with previous levels

These issues were commonly reported issues with the previous level of the specification.

More powerful timing function syntax is a common request from developers. See, for example: 2013 message or 2015 thread.

Developers frequently have to trigger style flushes in order to force transitions to start. It would be good to have an API that would avoid this requirement. See, for example, 2011 proposal.

Issues deferred from previous levels of the spec

These issues were in previous levels of the specification, but may not turn out to be important in this level either.

We may ultimately want to support a keypath syntax for the 'transition-property' property. A keypath syntax would enable different transitions to be specified for components of a property. For example the blur of a shadow could have a different transition than the color of a shadow.