CSS Transitions Level 2

Editor’s Draft,

More details about this document
This version:
https://drafts.csswg.org/css-transitions-2/
Issue Tracking:
CSSWG Issues Repository
Bugzilla bugs for all levels
Inline In Spec
Editors:
L. David Baron (Mozilla)
(Invited Expert)
Suggest an Edit for this Spec:
GitHub Editor

Abstract

CSS Transitions allows property changes in CSS values to occur smoothly over a specified duration.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Status of this document

This is a public copy of the editors’ draft. It is provided for discussion only and may change at any moment. Its publication here does not imply endorsement of its contents by W3C. Don’t cite this document other than as work in progress.

Please send feedback by filing issues in GitHub (preferred), including the spec code “css-transitions-2” in the title, like this: “[css-transitions-2] …summary of comment…”. All issues and comments are archived. Alternately, feedback can be sent to the (archived) public mailing list www-style@w3.org.

This document is governed by the 2 November 2021 W3C Process Document.

1. 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.

2. Transitions

2.1. The transition-property Property

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).

2.2. The transition-duration Property

The transition-duration property specifies the iteration duration of the transition’s associated animation effect.

2.3. The transition-timing-function Property

The transition-timing-function property specifies the timing function of the transition’s associated animation effect (see Web Animations 1 § 4.10 Time transformations).

Note: Unlike CSS animations, the timing function for CSS transitions applies to the animation effect as opposed to the individual keyframes since this allows it to be reflected in the transformed progress as used when calculating the reversing shortening factor.

2.4. The transition-delay Property

The transition-delay property specifies the start delay of the transition’s associated animation effect.

3. Starting of transitions

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. A transition may be disassociated from its owning element such that it has no owning element.

The set of running transitions includes only those transitions that have an owning element.

Whenever an implementation is required to cancel a transition, it must disassociate the transition from its owning element and run the procedure to cancel an animation on the transition.

Although not explicitly defined by level 1 of this specification, in addition to canceling transitions on elements that are no longer connected, implementations must also cancel any running transitions on elements that are no longer being rendered and remove transitions on them from the set of completed transitions.

Note that calling the cancel() method on the CSSTransition object representing a running transition does not cause the transition to be disassociated from its owning element.

3.1. Faster reversing of interrupted transitions

The reversing shortening factor and reversing-adjusted start value associated with a transition in order to produce correct reverse behavior, are associated with the transition itself and not, for example, its animation effect. As a result, transitions will use these same values when producing a reversed transition, even if the transition’s associated animation effect has been updated or replaced using the Web Animations API [WEB-ANIMATIONS].

3.2. The current transition generation

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.

4. Application of transitions

4.1. Animation composite order

Animations generated from the markup 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.

  2. Otherwise, if only one of A or B has an owning element, let the animation with an owning element sort first.

  3. 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

    • ::marker

    • ::before

    • any other pseudo-elements not mentioned specifically in this list, sorted in ascending order by the Unicode codepoints that make up each selector

    • ::after

    • element children

  4. Otherwise, if A and B have different transition generation values, sort by their corresponding transition generation in ascending order.

  5. 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.

4.2. Animation cascade level

Animations with an animation class of ‘CSS Transition’ that have an owning element are applied to the ‘Transitions declaration’ level of the CSS cascade. All other animations generated by the markup defined in this specification, including animations that no longer have an owning element, are applied to the ‘Animation declarations’ level of the cascade. (See Web Animations 1 § 5.4.5 Applying the composited result.)

5. Transition Events

5.1. 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.

The target for a transition event is the transition’s owning element. If there is no owning element, no transition events are dispatched (although the animation playback events defined in Web Animations are still dispatched at the corresponding CSSTransition object).

To avoid firing redundant events, the set of events to dispatch is based on comparing the phase of the transition in the previous animation frame to its current state.

The transition phase of a transition is initially ‘idle’ and is updated on each animation frame according to the first matching condition from below:

If the transition has no associated 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 has a pending play task or a pending pause task and its phase was previously ‘idle’ or ‘pending’,
The transition phase is ‘pending’.
Otherwise,
The transition phase is the phase of its associated effect.

For calculating the elapsedTime of each event, the following definitions are used:

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 associated effect.

Each time a new animation frame is established, the events to dispatch are determined by comparing the transition phase in the previous and current animation frame 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 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:

The typical sequences of events, then, are as follows:

6. DOM Interfaces

6.1. The CSSTransition interface

CSSTransition

In all current engines.

Firefox75+Safari13.1+Chrome78+
Opera?Edge79+
Edge (Legacy)NoneIENone
Firefox for Android?iOS Safari?Chrome for Android?Android WebView?Samsung Internet?Opera Mobile?
[Exposed=Window]
interface CSSTransition : Animation {
  readonly attribute CSSOMString transitionProperty;
};

CSSTransition/transitionProperty

In all current engines.

Firefox75+Safari13.1+Chrome84+
Opera?Edge84+
Edge (Legacy)?IENone
Firefox for Android?iOS Safari?Chrome for Android?Android WebView?Samsung Internet?Opera Mobile?

transitionProperty, of type CSSOMString, readonly

The expanded transition property name of this transition.

6.2. 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 opacity property.

The first time this occurs, calling 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 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'

7. 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.

8. 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.

Conformance

Document conventions

Conformance requirements are expressed with a combination of descriptive assertions and RFC 2119 terminology. The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in the normative parts of this document are to be interpreted as described in RFC 2119. However, for readability, these words do not appear in all uppercase letters in this specification.

All of the text of this specification is normative except sections explicitly marked as non-normative, examples, and notes. [RFC2119]

Examples in this specification are introduced with the words “for example” or are set apart from the normative text with class="example", like this:

This is an example of an informative example.

Informative notes begin with the word “Note” and are set apart from the normative text with class="note", like this:

Note, this is an informative note.

Advisements are normative sections styled to evoke special attention and are set apart from other normative text with <strong class="advisement">, like this: UAs MUST provide an accessible alternative.

Tests

Tests relating to the content of this specification may be documented in “Tests” blocks like this one. Any such block is non-normative.


Conformance classes

Conformance to this specification is defined for three conformance classes:

style sheet
A CSS style sheet.
renderer
A UA that interprets the semantics of a style sheet and renders documents that use them.
authoring tool
A UA that writes a style sheet.

A style sheet is conformant to this specification if all of its statements that use syntax defined in this module are valid according to the generic CSS grammar and the individual grammars of each feature defined in this module.

A renderer is conformant to this specification if, in addition to interpreting the style sheet as defined by the appropriate specifications, it supports all the features defined by this specification by parsing them correctly and rendering the document accordingly. However, the inability of a UA to correctly render a document due to limitations of the device does not make the UA non-conformant. (For example, a UA is not required to render color on a monochrome monitor.)

An authoring tool is conformant to this specification if it writes style sheets that are syntactically correct according to the generic CSS grammar and the individual grammars of each feature in this module, and meet all other conformance requirements of style sheets as described in this module.

Partial implementations

So that authors can exploit the forward-compatible parsing rules to assign fallback values, CSS renderers must treat as invalid (and ignore as appropriate) any at-rules, properties, property values, keywords, and other syntactic constructs for which they have no usable level of support. In particular, user agents must not selectively ignore unsupported component values and honor supported values in a single multi-value property declaration: if any value is considered invalid (as unsupported values must be), CSS requires that the entire declaration be ignored.

Implementations of Unstable and Proprietary Features

To avoid clashes with future stable CSS features, the CSSWG recommends following best practices for the implementation of unstable features and proprietary extensions to CSS.

Non-experimental implementations

Once a specification reaches the Candidate Recommendation stage, non-experimental implementations are possible, and implementors should release an unprefixed implementation of any CR-level feature they can demonstrate to be correctly implemented according to spec.

To establish and maintain the interoperability of CSS across implementations, the CSS Working Group requests that non-experimental CSS renderers submit an implementation report (and, if necessary, the testcases used for that implementation report) to the W3C before releasing an unprefixed implementation of any CSS features. Testcases submitted to W3C are subject to review and correction by the CSS Working Group.

Further information on submitting testcases and implementation reports can be found from on the CSS Working Group’s website at http://www.w3.org/Style/CSS/Test/. Questions should be directed to the public-css-testsuite@w3.org mailing list.

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

[CSS-BACKGROUNDS-3]
Bert Bos; Elika Etemad; Brad Kemper. CSS Backgrounds and Borders Module Level 3. URL: https://w3c.github.io/csswg-drafts/css-backgrounds/
[CSS-CASCADE-5]
Elika Etemad; Miriam Suzanne; Tab Atkins Jr.. CSS Cascading and Inheritance Level 5. URL: https://w3c.github.io/csswg-drafts/css-cascade-5/
[CSS-EASING-2]
CSS Easing Functions Level 2 URL: https://drafts.csswg.org/css-easing/
[CSS3-TRANSITIONS]
David Baron; et al. CSS Transitions. URL: https://w3c.github.io/csswg-drafts/css-transitions/
[CSSOM-1]
Daniel Glazman; Emilio Cobos Álvarez. CSS Object Model (CSSOM). URL: https://w3c.github.io/csswg-drafts/cssom/
[DOM]
Anne van Kesteren. DOM Standard. Living Standard. URL: https://dom.spec.whatwg.org/
[HTML]
Anne van Kesteren; et al. HTML Standard. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://datatracker.ietf.org/doc/html/rfc2119
[WEB-ANIMATIONS]
Brian Birtles; et al. Web Animations. URL: https://w3c.github.io/csswg-drafts/web-animations-1/
[WEBIDL]
Edgar Chen; Timothy Gu. Web IDL Standard. Living Standard. URL: https://webidl.spec.whatwg.org/

Informative References

[CSS-COLOR-4]
Tab Atkins Jr.; Chris Lilley; Lea Verou. CSS Color Module Level 4. URL: https://w3c.github.io/csswg-drafts/css-color/
[WEB-ANIMATIONS-2]
Web Animations Module Level 2 URL: https://drafts.csswg.org/web-animations-2/

IDL Index

[Exposed=Window]
interface CSSTransition : Animation {
  readonly attribute CSSOMString transitionProperty;
};

Issues Index

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.

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.