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.
Status of this document
This section describes the status of this document at the time of
its publication. Other documents may supersede this document. A list of
current W3C publications and the latest revision of this technical report
can be found in the W3C technical reports
index at http://www.w3.org/TR/.
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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mailing list www-style@w3.org (see
instructions) is preferred
for discussion of this specification. When sending e-mail, please put the
text “css-will-change” in the subject, preferably like this:
“[css-will-change] …summary of comment…”
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.
For example, when using CSS 3D Transforms to move an element around the screen,
the element and its contents might be promoted to a “layer”,
where they can render independently from the rest of the page and be composited in later.
This isolates the rendering of the content so that the rest of the page doesn’t have to be rerendered
if the element’s transform is the only thing that changes between frames,
and often provides significant speed benefits.
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.
2
Hinting at Future Behavior: the will-change property
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:
auto
Expresses no particular intent;
the user agent should apply whatever heuristics and optimizations it normally does.
scroll-position
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.
contents
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.
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.
For example, setting opacity to any value other than 1 creates a stacking context on the element.
Thus, setting will-change: opacity also creates a stacking context,
even if opacity is currently still equal to 1.
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.
Different browsers can use the information from will-change in different ways,
and even a single browser might use it in different ways at different time.
For example, a browser that promotes elements to their own “GPU layer”
when they have will-change: transform specified
might avoid doing that when there are too many elements declaring that,
to avoid exhausting GPU memory.
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
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.
Conformance classes
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.
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.