This specification is obsolete and has been replaced by the document at http://dev.w3.org/csswg/css-will-change/.
Do not attempt to implement this specification.
Do not refer to this specification except as a historical artifact.
This document defines the will-change 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 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.
GitHub Issues are preferred for discussion of this specification.
When filing an issue, please put the text “css-will-change” in the title,
preferably like this:
“[css-will-change] …summary of comment…”.
All issues and comments are archived,
and there is also a historical archive.
This document was produced by a group operating under
the 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.
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.
scroll-position
Indicates that the author expects to animate or change the scroll position of the element in the near future.
contents
Indicates that the author expects to animate or change something about the element’s contents in the near future.
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, 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.
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:
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.
Requirements for Responsible Implementation of CSS
The following sections define several conformance requirements
for implementing CSS responsibly,
in a way that promotes interoperability in the present and future.
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 property 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
Once a specification reaches the Candidate Recommendation stage,
implementers should release an unprefixed implementation
of any CR-level feature they can demonstrate
to be correctly implemented according to spec,
and should avoid exposing a prefixed variant of that feature.
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.