This document defines the will-change CSS property, which allows an author to inform the UA ahead of time of what kinds of changes they are likely to make to an element. This allows the UA to optimize how they handle the element ahead of time, performing potentially-expensive work preparing for an animation before the animation actually begins. CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, in speech, etc.
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This document is a First Public Working Draft.
Publication as a First Public Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
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Modern CSS renderers perform a number of complex optimizations in order to render webpages quickly and efficiently. Unfortunately, employing these optimizations often has a non-trivial start-up cost, which can have a negative impact on the responsiveness of a page.
However, setting up the element in a fresh layer is a relatively expensive operation, which can delay the start of a transform animation by a noticeable fraction of a second.
The will-change property defined in this specification allows an author to declare ahead-of-time what properties are likely to change in the future, so the UA can set up the appropriate optimizations some time before they’re needed. This way, when the actual change happens, the page updates in a snappy manner.
|Value:||auto | <animateable-feature>#|
|Applies to:||all elements|
|Computed value:||specified value|
<animateable-feature> = scroll-position | contents | <custom-ident>
The will-change property provides a rendering hint to the user agent, stating what kinds of changes the author expects to perform on the element. This allows the user agent to perform ahead-of-time any optimizations necessary for rendering those changes smoothly, avoiding “jank” when the author does begin changing or animating that feature.
Values have the following meanings:
Expresses no particular intent; the user agent should apply whatever heuristics and optimizations it normally does.
Indicates that the author expects to animate or change the scroll position of the element in the near future.
For example, browsers often only render the content in the "scroll window" on a scrollable element, and some of the content past that window, balancing memory and time savings from the skipped rendering against making scrolling look nice. A browser might take this value as a signal to expand the range of content around the scroll window that is rendered, so that longer/faster scrolls can be done smoothly.
Indicates that the author expects to animate or change something about the element’s contents in the near future.
For example, browsers often “cache” rendering of elements over time, because most things don’t change very often, or only change their position. However, if an element *does* change its contents regularly, producing and maintaining this cache is a waste of time. A browser might take this value as a signal to cache less aggressively on the element, or avoid caching at all and just continually re-render the element from scratch.
Indicates that the author expects to animate or change the property with the given name on the element in the near future.
The <custom-ident> production used here excludes the keywords will-change, none, all, auto, scroll-position, and contents, in addition to the keywords normally excluded from <custom-ident>.
Note: Note that most properties will have no effect when specified, as the user agent doesn’t perform any special optimizations for changes in most properties. It is still safe to specify them, though; it’ll simply have no effect.
If any non-initial value of a property would create a stacking context on the element, specifying that property in will-change must create a stacking context on the element.
If any non-initial value of a property would cause the element to generate a containing block for fixed-position elements, specifying that property in will-change must cause the element to generate a containing block for fixed-position elements.
If a non-initial value of a property would cause rendering differences on the element (such as using a different anti-aliasing strategy for text), the user agent should use that alternate rendering when the property is specified in will-change, to avoid sudden rendering differences when the property is eventually changed.
For example, browsers often handle elements with transform set to a non-initial value very differently from normal elements, perhaps rendering them to their own “GPU layer” or using other mechanisms to make it easier to quickly make the sort of transformations that transform can produce. A browser might take a value of transform as a signal that it should go ahead and promote the element to its own layer immediately, before the element starts to be transformed, to avoid any delay involved in rerendering the old and new layers.
The will-change property has no direct effect on the element it is specified on, beyond the creation of stacking contexts and containing blocks as specified above. It is solely a rendering hint to the user agent, allowing it set up potentially-expensive optimizations for certain types of changes before the changes actually start occurring.
Authors should avoid overusing this property, and shouldn’t apply it to an element unless it is known (or expected) that the element will change in the indicated way soon. For example, applying will-change to an element from a static CSS stylesheet is probably an error; most of the time, will-change should be applied by scripting, some time shortly before starting an animation or other change, and should be promptly reset to its initial value of auto when the element stops changing.
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
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.
Conformance to this specification is defined for three conformance classes:
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.
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.
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.
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 firstname.lastname@example.org mailing list.
|Name||Value||Initial||Applies to||Inh.||%ages||Media||Computed value|
|will-change||auto | <animateable-feature>#||auto||all elements||no||n/a||all||specified value|