csswg-drafts/css-will-change/Overview.bs at 357fe0dd5219f154b3e3e8f29cc44cf9799f800a · w3c/csswg-drafts · GitHub

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<h1>CSS Will Change Module Level 1</h1>

<pre class='metadata'>
Shortname: css-will-change
Level: 1
Status: ED
Group: CSSWG
ED: http://dev.w3.org/csswg/css-will-change/
TR: http://www.w3.org/TR/css-will-change/
Abstract: 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.
Editor: Tab Atkins Jr., Google Inc., http://xanthir.com/contact/
Link Defaults: css-transforms-1 (property) transform
</pre>

<h2 id='intro'>
Introduction</h2>

	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.

	<div class='example'>
		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.
	</div>

	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.

<h3 id='using'>
Using 'will-change' Well</h3>

	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.

<h4 id='dont-global' class='no-num no-toc'>
Don't Spam 'will-change' Across Too Many Properties or Elements</h4>

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

	<pre>* { will-change: all; }</pre>

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

	Wrong.  The browser <em>already</em> 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 <strong>some</strong> side-effects,
	and it's very unlikely that pages actually want all those side-effects on every element.

<h4 id='css-sparingly' class='no-num no-toc'>
Use 'will-change' Sparingly In Stylesheets</h4>

	Using 'will-change' directly in a stylesheet
	This is <em>usually</em> not what you actually mean;
	instead, 'will-change' should usually be flipped on and off via scripting
	before and after the change occurs (see <a section href="#dont-waste"></a>).
	However, there are some common circumstances in which it is appropriate to use 'will-change' directly in a stylesheet.

	<div class='example'>
		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:

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

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

	<div class='example'>
		Sometimes an element really <em>does</em> 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.

		<pre>
		.cats-flying-around-the-screen {
			will-change: left, top;
		}
		</pre>
	</div>

<h4 id='give-time' class='no-num no-toc'>
Give 'will-change' Sufficient Time To Work</h4>

	Another common bad pattern is to apply 'will-change' to an element
	<em>immediately</em> 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' <em>then</em>.

	<div class='example'>
		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:

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

		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:

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

<h4 id='dont-waste' class='no-num no-toc'>
Don't Waste Resources On Elements That Have Stopped Changing</h4>

	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 <em>remove</em> it after the element is done changing,
	so the browser can recover whatever resources the optimizations are claiming.


<h2 id='will-change'>
Hinting at Future Behavior: the 'will-change' property</h2>

	<pre class='propdef'>
	Name: will-change
	Value: auto | <<animateable-feature>>#
	Initial: auto
	Applies to: all elements
	Inherited: no
	Percentages: n/a
	Media: all
	Computed value: specified value
	</pre>

	<pre class='prod'><dfn>&lt;animateable-feature></dfn> = scroll-position | contents | <<custom-ident>></pre>

	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.

	<div class='note'>
		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 <em>too many</em> elements declaring that,
		to avoid exhausting GPU memory.
	</div>

	Values have the following meanings:

	<dl dfn-for="will-change" dfn-type="value">
		<dt><dfn>auto</dfn>
		<dd>
			Expresses no particular intent;
			the user agent should apply whatever heuristics and optimizations it normally does.

		<dt><dfn>scroll-position</dfn>
		<dd>
			Indicates that the author expects to animate or change the scroll position of the element in the near future.

			<p class='example'>
				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.

		<dt><dfn>contents</dfn>
		<dd>
			Indicates that the author expects to animate or change something about the element's contents in the near future.

			<div class='example'>
				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 <em>does</em> 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.
			</div>

			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.

		<dt><dfn><<custom-ident>></dfn>
		<dd>
			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.

			<p class='example'>
				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 <css>will-change</css>, <css>none</css>, <css>all</css>, ''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 <em>safe</em> to specify them, though;
			it'll simply have no effect.

			<p class='example'>
				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 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.

			<p class='example'>
				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 <em>currently</em> still equal to ''1''.
	</dl>

	The 'will-change' property has no <em>direct</em> 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.

Acknowledgements {#acks}
========================

Thanks to Benoit Gerard for originally suggesting the <css>will-animate</css> property,
and doing a lot of the initial design work.