CSS Will Change Module Level 1

1. Introduction

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.

1.1. Value Definitions

This specification follows the CSS property definition conventions from [CSS2] using the value definition syntax from [CSS-VALUES-3]. Value types not defined in this specification are defined in CSS Values & Units [CSS-VALUES-3]. Combination with other CSS modules may expand the definitions of these value types.

In addition to the property-specific values listed in their definitions, all properties defined in this specification also accept the CSS-wide keywords as their property value. For readability they have not been repeated explicitly.

1.2. Using will-change Well

The will-change property, like all performance hints, can be somewhat difficult to learn how to use “properly”, particularly since it has very little, if any, effect an author can directly detect. However, there are several simple “Dos and Don’ts” which hopefully will help develop a good intuition about how to use will-change well.

Don’t Spam will-change Across Too Many Properties or Elements

A common initial response to seeing will-change is to assume that code like this is a good idea:

* { will-change: transform, opacity /* , ... */; }

After all, this tells the browser to go ahead and optimize everything, which has to be good right?

Wrong. The browser already tries as hard as it can to optimize everything. Telling it to do so explicitly doesn’t help anything, and in fact has the capacity to do a lot of harm; some of the stronger optimizations that are likely to be tied to will-change end up using a lot of a machine’s resources, and when overused like this can cause the page to slow down or even crash.

In addition, will-change does have some side-effects, and it’s very unlikely that pages actually want all those side-effects on every element.

Use will-change Sparingly In Stylesheets

Using will-change directly in a stylesheet implies that the targeted elements are always a few moments away from changing. This is usually not what you actually mean; instead, will-change should usually be flipped on and off via scripting before and after the change occurs (see Don’t Waste Resources On Elements That Have Stopped Changing). However, there are some common circumstances in which it is appropriate to use will-change directly in a stylesheet.

For example, specifying will-change for a small number of persistent UI elements in a page which should react snappily to the user is appropriate:

body > .sidebar {
   will-change: transform;
   /* Will use 'transform' to slide it out
      when the user requests. */
}

Because this is limited to a small number of elements, the fact that the optimization is rarely actually used doesn’t hurt very much.

Sometimes an element really does change a property nearly constantly. Perhaps it responds to the user’s mouse movements, or just regularly takes some action that causes an animation. In this case, just declaring the will-change value in the stylesheet is fine, as it accurately describes that the element will regularly/constantly change, and so should be kept optimized.

.cats-flying-around-the-screen {
  will-change: left, top;
}

Give will-change Sufficient Time To Work

Another common bad pattern is to apply will-change to an element immediately before starting the animation or property change that it’s meant to help with. Unfortunately, most of those optimizations need time to be applied, and so they don’t have enough time to set-up when this is done, and the will-change has little to no effect. Instead, find some way to predict at least slightly ahead of time that something will change, and set will-change then.

For example, if an element is going to change when a user clicks on it, setting will-change on hover will usually give at least 200 milliseconds for the optimizations to be set up, as human reaction time is relatively slow. This can be done either via scripting, or rather simply with a CSS rule:

.element { transition: opacity .2s; opacity: 1; }
.element:hover { will-change: opacity; }
.element:active { opacity: .3; }

However, a rule like that is useless if the effect is going to happen on hover. In cases like these, it is often still possible to find some way to predict the action before it occurs. For example, hovering an ancestor may give enough lead time:

.element { transition: opacity .2s; opacity: 1; }
.container:hover > .element { will-change: opacity; }
.element:hover { opacity: .3; }

Don’t Waste Resources On Elements That Have Stopped Changing

Because the optimizations browsers use for changing some properties are expensive, browsers remove them and revert to normal behavior as soon as they can in normal circumstances. However, will-change will generally override this behavior, maintaining the optimizations for much longer than the browser would otherwise do.

As such, whenever you add will-change to an element, especially via scripting, don’t forget to remove it after the element is done changing, so the browser can recover whatever resources the optimizations are claiming.

2. Hinting at Future Behavior: the will-change property

Name:	will-change
Value:	auto \| <animateable-feature>#
Initial:	auto
Applies to:	all elements
Inherited:	no
Percentages:	n/a
Computed value:	specified value
Canonical order:	per grammar
Animation type:	not animatable

<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.

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.

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 continually, 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.

This value is mostly intended to help browsers optimize JS-based animations of content, which change aspects of an element’s contents many times per second. This kind of optimization, when possible, is already done automatically by browsers when declarative animations are used.

Note: This value more-or-less applies to the entire subtree of the element its declared on, as it indicates the browser should count on *any* of the descendants changing in some way. Using this on an element “high up” in your document might be very bad for your page’s performance; try to only use this on elements near the “bottom” of your document tree, containing as little of the document as possible.

<custom-ident>

If the <custom-ident> is an ASCII case-insensitive match for the name of a built-in CSS property, it indicates that the author expects to animate or change the property with the given name on the element in the near future. If the property given is a shorthand, it indicates the expectation for all the longhands the shorthand expands to.

For example, setting will-change: background; is identical to setting will-change: background-image, background-position, ... for all the properties that background expands into.

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.

Specifying a custom property must have no effect, which means that effects that happen through custom properties do not count for the rules below that are conditioned on any non-initial value of a property causing something.

Note: Specifying a value that’s not recognized as a property is fine; it simply has no effect. This allows you to safely specify new properties that exist in some user agents without negatively affecting down-level user agents that don’t know about that property.

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.

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 absolutely positioned elements, specifying that property in will-change must cause the element to generate a containing block for absolutely positioned elements.

If any non-initial value of a property would cause the element to generate a containing block for fixed positioned elements, specifying that property in will-change must cause the element to generate a containing block for fixed positioned elements.

If any 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.

3. Security Considerations

No Security concerns have been raised against this document

4. Privacy Considerations

No Privacy concerns have been raised against this document

5. Acknowledgements

Thanks to Benoit Girard for originally suggesting the will-animate property, and doing a lot of the initial design work.

6. Changes

Since the 03 December 2015 CR:

Added Security and Privacy sections
Clarified that unknown values are fine, and have no effect
Specified that ASCII Case-Insensitive matching is used against property names
Changed the animation type of the will-change property to not animatable
Dropped the "Media:" entry from propdef tables, as with other CSS specifications
Minor editorial clarifications, markup improvements

Since the April 29 2014 Working Draft:

Added an explanatory section giving guidance on how to use will-change well.
Specified the behavior of shorthands

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:

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

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

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

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.

CSS Will Change Module Level 1

Abstract

Status of this document