css-highlight-api-1/Overview.bs

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<pre class=metadata>
Title: CSS Custom Highlight API Module Level 1
Level: 1
Shortname: css-highlight-api
Status: ED
Work Status: exploring
Group: csswg
ED: https://drafts.csswg.org/css-highlight-api-1/
TR: https://www.w3.org/TR/css-highlight-api-1/
Previous Version: https://www.w3.org/TR/2020/WD-css-highlight-api-1-20201022/
Previous Version: https://www.w3.org/TR/2020/WD-css-highlight-api-1-20201208/
Editor: Florian Rivoal, On behalf of Bloomberg, https://florian.rivoal.net/, w3cid 43241
Editor: Sanket Joshi, Microsoft Corporation https://www.microsoft.com, https://github.com/sanketj, w3cid 115721
Editor: Megan Gardner, Apple Inc. https://apple.com/, w3cid 110930
Abstract:
	This CSS module describes a mechanism
	for styling arbitrary ranges of a document identified by script.
Complain About: accidental-2119 yes
</pre>

<pre class=link-defaults>
spec:css-color-4; type:property; text:color
spec:css-pseudo-4; type:dfn; text:highlight overlay
spec:dom; type:dfn; text:range
spec:css-values-4; type:dfn; text:identifier
</pre>

<style>
.sample-out {
	background: white;
	padding: 0.5ch;
	border: solid 1px;
	font-size: 2em;
}
</style>

<div class=non-normative>
<h2 id='intro'>
Introduction</h2>

	<em>This section is non-normative</em>.

	The Custom Highlight API extends the concept of [=highlight pseudo-elements=] (see [[css-pseudo-4#highlight-pseudos]])
	by providing a way for web developers to style the text
	of arbitrary Range objects,
	rather than being limited to the user agent defined
	''::selection'',
	''::inactive-selection'',
	''::spelling-error'',
	and '''::grammar-error'''.
	This is useful in a variety of scenarios,
	including editing frameworks that wish to implement their own selection,
	find-on-page over virtualized documents,
	multiple selection to represent online collaboration,
	or spellchecking frameworks.

	The Custom Highlight API provides a programmatic way of adding and removing highlights
	that do not affect the underlying DOM structure,
	but instead applies styles to text based on [=range=] objects,
	accessed via the ''::highlight()'' pseudo element.

	<div id=intro-ex class=example>
		The following code uses the ''::highlight()'' pseudo-element
		to apply a yellow background and blue foreground color to the text <q>One two</q>.
		It does so by adding a {{Highlight}} to the {{HighlightRegistry}}
		(both of these are new concepts introduced by this specification).
		The {{Highlight}} will contain a {{Range}} whose boundary points surround the text <q>One two</q>.

		<xmp highlight=html>
			<style>
				:root::highlight(example-highlight) {
					background-color: yellow;
					color: blue;
				}
			</style>
			<body><span>One </span><span>two </span><span>three…</span>
			<script>
				let r = new Range();
				r.setStart(document.body, 0);
				r.setEnd(document.body, 2);

				CSS.highlights.set("example-highlight", new Highlight(r));
			</script>
		</xmp>

		The result would look like:
		<div class=sample-out>
			<span style="background-color:yellow;color:blue;">One Two </span>three…
		</div>
	</div>
</div>

<h2 id="interaction">
Module Interactions</h2>

	This module depends on the Infra Standard [[!INFRA]]
	and on WebIDL [[!WebIDL]].

	It assumes general familiarity with CSS
	and with the DOM Standard [[DOM]],
	and specifically extends the mechanisms defined in CSS Pseudo-Elements Module Level 4 [[!css-pseudo-4]]
	to handle [=highlight pseudo-elements=].
	The Selectors Level 4 [[!selectors-4]] specification defines how [=pseudo-elements=] work in general.

	See [[#references]] for a full list of dependencies.

	Note: This draft is an early version.
	As it matures, the CSS-WG could decide to keep it as an independent module,
	or might prefer to fold it into [[css-pseudo-4]],
	or a later version of that module.

<h2 id=highlights-set-up>
Setting up Custom Highlights</h2>

<h3 id=creation>
Creating Custom Highlights</h3>

	A <dfn export>custom highlight</dfn> is a collection of [=ranges=]
	representing portions of a document.
	They do not necessarily fit into the element tree,
	and can arbitrarily cross element boundaries without honoring its nesting structure.
	They can be used to affect the appearance of these portions of the document
	(see [[#styling-highlights]]),
	or to handle to events associated with them
	(see [[#events]]).

	[=Custom highlights=] are represented by
	<dfn interface>Highlight</dfn> objects,
	[=setlike=] objects whose [=set entries=] are {{AbstractRange}} objects.
	[=Ranges=] can be added to a [=custom highlight=]
	either by passing them to its constructor,
	or by using the usual API of [=setlike=] objects
	to manipulate its [=set entries=].

	Note: As the [=ranges=] in a [=custom highlight=] are {{AbstractRange}} objects,
	authors can chose between using {{Range}} objects and {{StaticRange}} objects.
	See [[#range-invalidation]] for more details about this choice and its implications.

	Note: When creating {{Range}} objects for use in [=custom highlights=], it is
	suggested that authors avoid placing [=range=] endpoints in the middle of a [=grapheme=],
	such as when a visible unit of text is comprised of multiple [=Unicode code points=].
	Doing so can create undesirable highlighting effects, such as highlighting only part of
	an Indic syllable. In addition, care needs to be taken to avoid placing an endpoint
	in the middle of a [=supplementary character=].

	<xmp class="idl">
	enum HighlightType {
		"highlight",
		"spelling-error",
		"grammar-error"
	};

	[Exposed=Window]
	interface Highlight {
		constructor(AbstractRange... initialRanges);
		setlike<AbstractRange>;
		attribute long priority;
		attribute HighlightType type;
	};
	</xmp>

	See [[#priorities]] for more information on the {{Highlight/priority}} attribute.

	See [[#highlight-types]] for more information on the {{Highlight/type}} attribute.

	<div algorithm="to create a custom highlight">
		When the <dfn for=Highlight constructor>Highlight(AbstractRange... initialRanges)</dfn> constructor is invoked,
		run the following steps:

		<ol>
			<li>
				Let |highlight| be the new {{Highlight}} object.
			<li>
				Set |highlight|'s {{Highlight/priority}} to <code>0</code>.
			<li>
				Set |highlight|'s {{Highlight/type}} to {{HighlightType/highlight}}.
			<li>
				For each |range| of {{initialRanges}},
				let |rangeArg| be the result of [=converted to an ECMAScript value|converting=] |range| to an ECMAScript value,
				then run [[webidl#js-set-add|the steps for a built-in setlike add function]],
				with |highlight| as the <code>this</code> value,
				and |rangeArg| as the argument.
			<li>
				Return |highlight|.
		</ol>
	</div>

<h3 id=registration>
Registering Custom Highlights</h3>

	In order to have any effect,
	[=custom highlights=] need to be
	[=registered=] into the [=highlight registry=].

	The <dfn>highlight registry</dfn> is accessed via the {{CSS/highlights}} attribute of the {{CSS}} namespace,
	and represents all the [=custom highlights=] [=registered=] for the [=current global object=]’s [=associated Document=].
	It is a [=maplike=], and can be updated using the usual methods.
	It's [=map entries=] is initially empty.

	A [=custom highlight=] is said to be <dfn>registered</dfn>
	if it is in the [=highlight registry=].
	It stops being [=registered=] if it is later removed.

	<xmp class="idl">
	partial namespace CSS {
		readonly attribute HighlightRegistry highlights;
	};

	[Exposed=Window]
	interface HighlightRegistry {
		maplike<DOMString, Highlight>;
	};
	</xmp>

	<div algorithm="to register a custom highlight">
		To [=register=] a [=custom highlight=],
		invoke the <code>set</code> method of the [=highlight registry=]
		which will run [[webidl#js-map-set|the steps for a built-in maplike set function]],
		with the [=highlight registry=] as the <code>this</code> value,
		the passed-in [=custom highlight name=] as <var ignore>keyArg</var>,
		and the passed-in highlight as <var ignore>valueArg</var>.

		The <dfn export>custom highlight name</dfn> assigned to a [=custom highlight=] when it is [=registered=]
		is used to identify the highlight during styling (see [[#styling-highlights]]).

	Note: When registering a [=custom highlight=],
	authors are advised to use a [=custom highlight name=] that is a valid CSS [=identifier=].
	Using a name that is not a valid identifier can make the highlight hard,
	and in some cases impossible, to style via CSS.

	Note: It is possible to [=register=] a [=custom highlight=] with more than one [=custom highlight name=].
	However, using more than one name to style a highlight
	will assign the highlight multiple different sets of styles,
	without a way to control the stacking order of conflicting styles
	within these sets during [[#painting|painting]].
	This could be limiting for authors and could cause confusing painting behavior
	(see the <a href="#styling-problems-with-multiple-names-per-highlight">example</a> below for more context).
	Therefore, <b>authors are advised to only use one name per highlight during styling</b>.

	<div class=example id=styling-problems-with-multiple-names-per-highlight>
		<xmp highlight=html>
			<style>
				div::highlight(bar) {
					color: red;
				}
				div::highlight(foo) {
					color: green;
				}
			</style>
			<body><div>abc</div>
			<script>
				let div = document.body.firstChild;
				let r = new Range();
				r.setStart(div, 0);
				r.setEnd(div, 1);
				let h = new Highlight(r);
				CSS.highlights.set('foo', h);
				CSS.highlights.set('bar', h);
			</script>
		</xmp>

		In the example above,
		the same [=custom highlight=] object is [=registered=] under the names <code>foo</code> and <code>bar</code>.
		Since each of the [=style rules=] target the same highlight and have the same [=specificity=],
		authors might expect the last rule to win in cascading order
		and the highlighted content to be green.
		However, each highlight name gets an independent set of highlight styles,
		and the highlight will be painted once per name.
		In this case, because <code>foo</code> was registered before <code>bar</code>,
		the highlight will be first painted with <code>foo</code>'s color (green)
		and then with <code>bar</code>'s color (red).
		As a result, the highlighted content will appear red.
	</div>

<h2 id=styling-highlights>
Styling Custom Highlights</h2>

<h3 id=custom-highlight-pseudo>
The Custom Highlight Pseudo-element: ''::highlight()''</h3>

	The ''::highlight()'' pseudo-element
	(also known as the <dfn>custom highlight pseudo-element</dfn>)
	represents the portion of a document that
	is being [=contained=] or [=partially contained=]
	in all the [=ranges=] of the [=registered=] [=custom highlight=]
	with the [=custom highlight name=] specified as its argument.

<h3 id=processing-model>
Processing Model</h3>

<h4 id=applicable-properties>
Applicable Properties</h4>

	[=Custom highlight pseudo-elements=],
	like the built-in [=highlight pseudo-elements=],
	can only be styled with a limited set of properties.
	See [[css-pseudo-4#highlight-styling]] for the full list.

<h4 id=default-styles>
Default Styles</h4>

	UAs must not define any default UA stylesheet rules
	or <a spec=css-pseudo>paired default highlight colors</a>
	for any [=custom highlight pseudo-elements=].
	In other words,
	when some content is highlighted by an unstyled custom highlight,
	its presentation must not change.

<h4 id=c-and-h>
Cascading and Inheritance</h4>

	The [=cascading=] and [=inheritance=] of [=custom highlight pseudo-elements=] is handled
	identically to that of the built-in [=highlight pseudo-elements=],
	as defined in [[css-pseudo-4#highlight-cascade]].

<h4 id=painting>
Painting</h4>

	The painting of [=custom highlights=] is also handled
	identically to that of the built-in [=highlight pseudo-elements=],
	as specified in
	[[css-pseudo-4#highlight-bounds]] and [[css-pseudo-4#highlight-painting]],
	with the following clarifications:

	<ul>
		<li>
			[=Collapsed=] [=ranges=] are not rendered.

		<li>
			Overlapping [=ranges=] within a single [=custom highlight=] are rendered
			as if a single range representing the union of the overlapping ones
			had been specified.

			<div class=example id=overlap-ranges-ex>
				The following example renders in a single highlight with semi-transparent blue background,
				not two overlapping ones which can be seen through each other.
				<xmp highlight=html>
					<style>
						::highlight(sample) { background-color: rgba(0, 0, 255, 0.3); }
					</style>
					<body>Lorem Ipsum.
					<script>
						let textNode = document.body.firstChild;

						let r1 = new Range();
						r1.setStart(textNode, 1);
						r1.setEnd(textNode, 5);

						let r2 = new Range();
						r2.setStart(textNode, 3);
						r2.setEnd(textNode, 7);

						CSS.highlights.set("sample", new Highlight(r1, r2));
					</script>
				</xmp>

				In other words, this rendering would be correct:
				<div class=sample-out>
					L<span style="background-color: rgba(0, 0, 255, 0.3)">orem I</span>psum.
				</div>

				However, this one would be incorrect:
				<div class=sample-out>
					L<span style="background-color: rgba(0, 0, 255, 0.3)">or<span style="background-color: rgba(0, 0, 255, 0.3)">em</span> I</span>psum.
				</div>
			</div>

		<li>
			The [=highlight overlays=] of the [=custom highlights=]
			are below those of the built-in [=highlight pseudo-elements=]
			in the stacking order described in [[css-pseudo-4#highlight-painting]].

		<li>
			The relative stacking order of the [=highlight overlays=]
			of multiple [=custom highlights=]
			is defined by their [=priority=]
			(see [[#priorities]]).
	</ul>

<h4 id=priorities>
Priority of Overlapping Highlights</h4>

	A [=custom highlight=]'s {{Highlight/priority}} attribute
	defines its <dfn>priority</dfn>.
	This is used to determine the stacking order of the corresponding [=highlight overlay=]
	during painting operations (see [[#painting]]).
	A higher [=priority=] results in being above in the stacking order.
	A custom highlight will have a default numerical priority of 0
	if its {{Highlight/priority}} attribute has not been explicitly set.

	When two or more [=custom highlights=] have the same numerical priority,
	the one most recently [=registered=] has the higher effective [=priority=].

	<div class=example id=overlap-highlight-ex>
		<xmp highlight=html>
			<style>
				:root::highlight(foo) {
					color:blue;
					background-color:yellow;
				}
				:root::highlight(bar) {
					background-color:orange;
				}
			</style>
			<body>Some text
			<script>
				let textNode = document.body.firstChild;

				let r1 = new Range();
				r1.setStart(textNode, 0);
				r1.setEnd(textNode, 6);

				let r2 = new Range();
				r2.setStart(textNode, 3);
				r2.setEnd(textNode, 9);

				let h1 = new Highlight(r1);
				let h2 = new Highlight(r2);

				CSS.highlights.set("foo", h1);
				CSS.highlights.set("bar", h2);
			</script>
		</xmp>

		As there are no priorities set
		(i.e. there is a tie between <code>h1</code> and <code>h2</code>),
		the custom highlights' styles are stacked
		in order of insertion into the [=highlight registry=].
		The rendered results will have "Som" with blue text on yellow background,
		"e t" with blue text on orange background,
		and "ext" with the default color on orange background.

		<div class=sample-out>
			<span style="background:yellow;color:blue;">Som</span><span style="background:orange;color:blue;">e t</span><span style="background:orange;">ext</span>
		</div>

		Setting <code highlight=javascript>h1.priority = 1;</code>
		would cause <code>h1</code> to stack higher than <code>h2</code>,
		which would result in "Some t" being blue on yellow,
		and "ext" being default color on orange.

		<div class=sample-out>
			<span style="background:yellow;color:blue;">Some t</span><span style="background:orange;">ext</span>
		</div>
	</div>

<h4 id=highlight-types>
Highlight types</h4>

	A [=custom highlight=]'s {{Highlight/type}} attribute is used by authors
	to specify the semantic meaning of the highlight.
	This allows assistive technologies to include this meaning
	when exposing the highlight to users.

	A custom highlight will have a default type of {{HighlightType/highlight}}
	if its {{Highlight/type}} attribute has not been explicitly set.

	Note: Authors are advised to set a [=custom highlight=]'s {{Highlight/type}} to {{HighlightType/spelling-error}}
	when that [=custom highlight=] is being used to emphasize misspelled content.
	Authors are advised to set a [=custom highlight=]'s {{Highlight/type}} to {{HighlightType/grammar-error}}
	when that [=custom highlight=] is being used to emphasize content that is grammatically incorrect.
	For all other use cases {{Highlight/type}} is best left as {{HighlightType/highlight}}.

	UAs should make [=custom highlight=]s available to assistive technologies.
	When exposing a highlight using a given platform accessibility API,
	UAs should expose the semantic meaning of the highlight
	as specified by its {{Highlight/type}} attribute
	with as much specificity as possible for that accessibility API.

	Note: For example,
	if a platform accessibility API has the capability to express spelling errors and grammar errors specifically,
	then UAs is expected to use these capabilities to convey the semantics for highlights
	with {{HighlightType/spelling-error}} and {{HighlightType/spelling-error}}.
	If an accessibility API only has the capability to express spelling errors,
	then UAs would be expected to convey both highlights with {{HighlightType/spelling-error}}
	and with {{HighlightType/grammar-error}} using spelling error semantics.
	If an accessibility API has support for expressing neither spelling errors nor grammar errors,
	then UAs would expose all highlights as generic {{HighlightType/highlight}} regardless of their actual {{Highlight/type}}.

	Note: This initial set of types was chosen
	because they are expected to be popular use cases for Highlight API
	and there is some existing support for expressing their semantics in platform accessibility APIs today.
	Accessibility APIs currently don't have any way to express the specific semantics
	of other expected Highlight API use cases.
	More types could later be added to {{HighlightType}}
	as accessibility APIs gain support for expressing additional popular use cases of Highlight API.

<h2 id=responding-to-changes>
Responding to Changes</h2>

<h3 id=repaint>
Repaints</h3>

	The addition or removal
	of a [=custom highlight=] in the [=highlight registry=],
	or of a [=range=] in a [=registered=] [=custom highlight=],
	must cause the user agent to reevaluate the rendering,
	and to repaint if appropriate.

	The user agent must also repaint highlights as needed
	in response to changes by the author
	to the {{Highlight/priority}},
	or to the [=boundary points=] of {{Range}}s
	of a [=registered=] [=custom highlight=].

	This repaint is asynchronous, and the APIs mentioned above must not block while waiting for the
	repaint to happen.

<h3 id=range-invalidation>
Range Updating and Invalidation</h3>

	Authors can build [=custom highlights=] using either {{Range}}s or {{StaticRange}}s.

	The resulting [=custom highlight=] represents the same parts of the document,
	and can be styled identically.
	However, the behavior is different
	in case the underlying document is modified.

	{{Range}}s are [=live ranges=].
	The user agent will adjust the [=boundary points=] of {{Range}}s
	in response to DOM changes overlapping the range or at its boundary,
	and [[#repaint|repaint]] accordingly.
	[=Boundary points=] of [=live ranges=] can also be changed
	by the author.

	On the other hand,
	the user agent must not adjust the [=boundary points=] of {{StaticRange}}s
	in response to DOM changes,
	nor can they be modified by the author after creation.
	The user agent is expected to store the actual {{StaticRange}}s, rather than backing
	them up with live {{Range}}s.

	<div class=advisement>
		Updating all {{Range}} objects as the DOM is modified
		has a significant performance cost.
		Authors who intend to observe DOM changes and react to them
		by adjusting or recreating the ranges in their [=custom highlights=]
		are strongly encouraged to use {{StaticRange}}s
		in order to avoid this costly but unnecessary step.

		Conversely, authors who use {{StaticRange}}s
		should observe and react to DOM changes,
		by discarding stale [=ranges=] or [=custom highlights=]
		and recreating new ones.
	</div>

	When computing how to render a document,
	if [=start node=] or [=end node=] of any [=range=]
	in the [=highlight registry=] associated with that document's window
	refer to a {{Node}} whose [=shadow-including root=] is not that document,
	the user agent must ignore that [=range=].
	If any {{StaticRange}} in the [=highlight registry=] associated with that document's window
	is not <a spec=dom for="StaticRange">valid</a>,
	the user agent must ignore that [=range=].

<h2 id=events>
Event Handling</h2>

	Issue: Section on Events TBD, based on https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/master/highlight/events-explainer.md

	Issue: should custom highlights have a dedicated event handling mechanism,
	or should that be added to pseudo-elements in general?

<div class=non-normative>
<h2 class="no-num" id=priv>
Appendix A. Privacy Considerations</h2>

	<em>This section is non-normative</em>.

	This specification is not thought to introduce any new privacy concern.
	Anyone suspecting that this is not accurate is encouraged
	to get in touch with the CSS Working Group or the co-editors.

<h2 class="no-num" id=sec>
Appendix B. Security Considerations</h2>

	<em>This section is non-normative</em>.

	This specification is not thought to introduce any new security concern.
	Anyone suspecting that this is not accurate is encouraged
	to get in touch with the CSS Working Group or the co-editors.

</div>

<div class=non-normative>
<h2 class="no-num" id="credits">
Appendix C. Acknowledgements</h2>

	<em>This section is non-normative</em>.

	Issue: Acknowledge people (other than editors) who deserve credit for this.
</div>

<div class=non-normative>
<h2 class="no-num" id="changes">
Appendix D. Changes</h2>

	<em>This section is non-normative</em>.

<h3 class="no-num" id="changes-since-20211215">
Changes since the <a href="https://www.w3.org/TR/2020/WD-css-highlight-api-1-20211215/">15 December 2021 Working Draft</a></h3>

	In addition to various editorial improvements and minor tweaks,
	the main changes are:

	* Specified that highlight repainting has to be done asynchronously.
		(See <a href="https://github.com/w3c/csswg-drafts/pull/6987">Issue 6987</a>)
	* Clarify that UAs cannot specify paired default highlight colors for custom highlights.
		(See <a href="https://github.com/w3c/csswg-drafts/pull/6665">Issue 6665</a>)
	* Clarify that there is no restriction on custom highlights crossing containment boundaries.
		(See <a href="https://github.com/w3c/csswg-drafts/issues/4598">Issue 4598</a>)
	* Added an I18N warning about placement of [=range=] endpoints in the middle of a
		[=grapheme=] or [=supplementary character=].

<h3 class="no-num" id="changes-since-20201208">
Changes since the <a href="https://www.w3.org/TR/2020/WD-css-highlight-api-1-20201208/">8 December 2020 Working Draft</a></h3>

	In addition to various editorial improvements and minor tweaks,
	the main changes are:

	* Renamed <code>HighlightsRegister</code> to {{HighlightRegistry}}
	* Removed the redundant <code>add()</code> method from {{HighlightRegistry}}.
		(See <a href="https://github.com/w3c/csswg-drafts/pull/6092">Issue 6092</a>)
	* Make custom highlight overlays stack below native highlight overlays.
		(See <a href="https://github.com/w3c/csswg-drafts/pull/4595">Issue 4595</a>)
	* Handle highlight priority with integers rather than floats.
		(See <a href="https://github.com/w3c/csswg-drafts/pull/4592">Issue 4592</a>)
	* Define the default value for highlight priority to be 0.
		(See <a href="https://github.com/w3c/csswg-drafts/pull/6136">Issue 6136</a>)
	* Made HighlightRegistry maplike (rather than setlike)
		and remove <code>name</code> property from Highlight.
		(See <a href="https://github.com/w3c/csswg-drafts/pull/5910">Issue 5910</a>)
	* Clarified that ranges from the wrong window are not painted.
		(See <a href="https://github.com/w3c/csswg-drafts/pull/6417">Issue 6417</a>)
	* Specify that custom highlights have no UA styles.
		(See <a href="https://github.com/w3c/csswg-drafts/pull/6375">Issue 6375</a>)
	* Deferred to the [[DOM]] specification for range invalidation
		(See <a href="https://github.com/w3c/csswg-drafts/pull/4597">Issue 4597</a>)
	* Added a {{Highlight/type}} attribute to {{Highlight}}
		to give clearer semantics to different highlights,
		in support of exposing highlights to accessibility tools.
		(See <a href="https://github.com/w3c/csswg-drafts/pull/6498">Issue 6498</a>)

<h3 class="no-num" id="changes-since-20201022">
Changes since the <a href="https://www.w3.org/TR/2020/WD-css-highlight-api-1-20201022/">22 October 2020 Working Draft</a></h3>

	There have been only editorial changes since the <a href="https://www.w3.org/TR/2020/WD-css-highlight-api-1-20201022/">22 October 2020 Working Draft</a>;
	see <a href="https://services.w3.org/htmldiff?doc1=https%3A%2F%2Fwww.w3.org%2FTR%2F2020%2FWD-css-highlight-api-1-20201022%2F&doc2=https%3A%2F%2Fwww.w3.org%2FTR%2Fcss-highlight-api-1%2F">diffs</a>.
</div>

	<!-- This text from the explainer seems useful, and should go somewhere. But where?
	----
	If there are DOM/CSS changes that result in a different cascaded highlight map for a given element,
	and there exists one or more Range objects in the highlight registry for the cascaded identifiers,
	the layout representation of that element should be notified that the painting of the element might have changed.

	Ranges that are positioned inside of documents that are not in the view are ignored.

	The HighlightRegistry is per-document — therefore, Ranges that are positioned inside of a different document than the HighlightRegistry it is a part of are ignored for rendering.
	-->