CSS Transitions Level 2

Editor’s Draft,

This version:
http://dev.w3.org/csswg/css-transitions-2/
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www-style@w3.org with subject line “[css-transitions-2] … message topic …” (archives)
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Inline In Spec
Editor:
(Mozilla)

Copyright © 2015 W3C® (MIT, ERCIM, Keio, Beihang). W3C liability, trademark and document use rules apply.

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, in speech, 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.

The (archived) public mailing list www-style@w3.org (see instructions) is preferred for discussion of this specification. When sending e-mail, please put the text “css-transitions-2” in the subject, preferably like this: “[css-transitions-2] …summary of comment…”

This document was produced by the CSS Working Group (part of the Style Activity).

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

This document is governed by the 1 August 2014 W3C Process Document.

Table of Contents

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. Transition Events

2.1. Types of TransitionEvent

The additional types of transition events that can occur are:

transitionstart
The transitionstart event occurs when a transition is started (i.e., when it is added to the set of running transitions).
  • Bubbles: Yes
  • Cancelable: No
  • Context Info: propertyName, elapsedTime, pseudoElement
transitioncancel
The transitioncancel event occurs when a transition is cancelled.
  • Bubbles: Yes
  • Cancelable: No
  • Context Info: propertyName, elapsedTime, pseudoElement

3. Starting of transitions

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. Web Animations Instantiation of CSS Animations

TBD. Not sure if we really want a separate 'web animations' section or if we should do a better job of integrating the two.

4.1. The 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).

4.2. Animation priority

The animations generated from the markup and interfaces (e.g. the CSSTransition constructor) defined in this specification have an animation type of ‘CSS Transition’.

CSS Transitions have a lower priority that CSS Animations and animations without a specific animation type.

Within the set of CSS Transitions, two animations A and B are sorted in priority order (lowest to highest) as follows:

  1. Define the owning element as the element or pseudo-element to which the transition-property property was applied that generated this CSS Transition. If the CSS Transition was generated directly by script (e.g. using the CSSTransition constructor) then it has no owning element.

  2. If neither A nor B has an owning element, sort based on A and B such that their animation sequence numbers are in ascending order.

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

  4. 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

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

  6. 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’).

5. DOM Interfaces

5.1. The CSSTransition interface

interface CSSTransition : Animation {
  readonly attribute DOMString transitionProperty;
};

transitionProperty, of type DOMString, readonly

The expanded transition property name of this transition.

TBD: Define a constructor for this

6. Issues deferred from previous levels

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.

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.

Experimental implementations

To avoid clashes with future CSS features, the CSS2.1 specification reserves a prefixed syntax for proprietary and experimental extensions to CSS.

Prior to a specification reaching the Candidate Recommendation stage in the W3C process, all implementations of a CSS feature are considered experimental. The CSS Working Group recommends that implementations use a vendor-prefixed syntax for such features, including those in W3C Working Drafts. This avoids incompatibilities with future changes in the draft.

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

[WebIDL]
Cameron McCormack. Web IDL. 19 April 2012. CR. URL: http://www.w3.org/TR/WebIDL/
[CSS-BACKGROUNDS-3]
CSS Backgrounds and Borders Module Level 3 URL: http://www.w3.org/TR/css3-background/
[CSS-TRANSITIONS-1]
CSS Transitions Module Level 1 URL: http://www.w3.org/TR/css3-transitions/
[CSS3-TRANSITIONS]
Dean Jackson; et al. CSS Transitions. 19 November 2013. WD. URL: http://www.w3.org/TR/css3-transitions/
[DOM-LS]
Document Object Model URL: http://dom.spec.whatwg.org/
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Best Current Practice. URL: https://tools.ietf.org/html/rfc2119
[WEB-ANIMATIONS]
Brian Birtles; et al. Web Animations 1.0. 5 June 2014. WD. URL: http://www.w3.org/TR/web-animations/
[WEB-ANIMATIONS-1]
Web Animations Level 1 URL: http://www.w3.org/TR/web-animations/

IDL Index

interface CSSTransition : Animation {
  readonly attribute DOMString transitionProperty;
};

Issues Index

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. ↵