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.
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.
A custom highlight 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 § 4 Styling Custom Highlights), or to handle to events associated with them (see § 7 Event Handling).
Custom highlights are represented by Highlight 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: 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.
To
registera
custom highlight, invoke the
set
method of the
highlight registrywhich will run
the steps for a built-in maplike set function, with the
highlight registryas the
this
value, the passed-in
custom highlight nameas
keyArg, and the passed-in highlight as
valueArg.
The custom highlight name assigned to a custom highlight when it is registered is used to identify the highlight during styling (see § 4 Styling Custom 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. This could be limiting for authors and could cause confusing painting behavior (see the example below for more context). Therefore, authors are advised to only use one name per highlight during styling.
<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>
In the example above, the same custom highlight object is registered under the names foo and bar. 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 foo was registered before bar, the highlight will be first painted with foo’s color (green) and then with bar’s color (red). As a result, the highlighted content will appear red.
The ::highlight() pseudo-element (also known as the custom highlight pseudo-element) 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.
4.2. Processing Model 4.2.1. Applicable PropertiesCustom highlight pseudo-elements, like the built-in highlight pseudo-elements, can only be styled with a limited set of properties. See CSS Pseudo-Elements 4 § 3.2 Styling Highlights for the full list.
4.2.2. Default StylesUAs must not define any default UA stylesheet rules or paired default highlight colors for any custom highlight pseudo-elements. In other words, when some content is highlighted by an unstyled custom highlight, its presentation must not change.
4.2.3. Cascading and InheritanceThe 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-Elements 4 § 3.5 Cascading and Per-Element Highlight Styles.
4.2.4. PaintingThe painting of custom highlights is also handled identically to that of the built-in highlight pseudo-elements, as specified in CSS Pseudo-Elements 4 § 3.4 Area of a Highlight and CSS Pseudo-Elements 4 § 3.6 Painting the Highlight, with the following clarifications:
The following example renders in a single highlight with semi-transparent blue background, not two overlapping ones which can be seen through each other.
<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>
In other words, this rendering would be correct:
Lorem Ipsum.
However, this one would be incorrect:
Lorem Ipsum.
A custom highlight’s priority attribute defines its priority. This is used to determine the stacking order of the corresponding highlight overlay during painting operations (see § 4.2.4 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 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.
<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>
As there are no priorities set (i.e. there is a tie between h1 and h2), 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.
Some text
Setting h1 . priority = 1 ; would cause h1 to stack higher than h2, which would result in "Some t" being blue on yellow, and "ext" being default color on orange.
Some text
4.2.6. Highlight typesA custom highlight’s 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 highlight if its type attribute has not been explicitly set.
Note: Authors are advised to set a custom highlight’s type to spelling-error when that custom highlight is being used to emphasize misspelled content. Authors are advised to set a custom highlight’s type to grammar-error when that custom highlight is being used to emphasize content that is grammatically incorrect. For all other use cases type is best left as highlight.
UAs should make custom highlights 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 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 spelling-error and spelling-error. If an accessibility API only has the capability to express spelling errors, then UAs would be expected to convey both highlights with spelling-error and with 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 highlight regardless of their actual 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.
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 priority, or to the boundary points of Ranges of a registered custom highlight.
This repaint is asynchronous, and the APIs mentioned above must not block while waiting for the repaint to happen.
5.2. Range Updating and InvalidationAuthors can build custom highlights using either Ranges or StaticRanges.
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.
Ranges are live ranges. The user agent will adjust the boundary points of Ranges in response to DOM changes overlapping the range or at its boundary, and 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 StaticRanges in response to DOM changes, nor can they be modified by the author after creation. The user agent is expected to store the actual StaticRanges, rather than backing them up with live Ranges.
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 highlightsare strongly encouraged to use
StaticRange
s in order to avoid this costly but unnecessary step.
Conversely, authors who use StaticRanges should observe and react to DOM changes, by discarding stale ranges or custom highlights and recreating new ones.
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 valid, the user agent must ignore that range.
The highlightsFromPoint(x, y, options) method allows developers to build scenarios involving user interaction with custom highlights. The method returns a sequence containing the custom highlights at a given x, y coordinate. The custom highlights are listed in this sequence in descending priority order. By default, custom highlights in a shadow tree are not returned, but the developer has the possibility to pass in an optional options dictionary with a shadowRoots property containing a sequence of ShadowRoot objects. Highlights contained within a shadow tree provided in this way will be returned.
The following example shows a way to use
highlightsFromPoint
to interact with mouse click
events.
<style>
:root::highlight(foo) {
background-color:yellow;
}
:root::highlight(bar) {
color:red;
}
</style>
<body>abc
<script>
document.addEventListener('click', (event) => {
const mouseX = event.clientX;
const mouseY = event.clientY;
console.log(CSS.highlights.highlightsFromPoint(mouseX, mouseY));
});
let textNode = document.body.firstChild;
let r1 = new Range();
r1.setStart(textNode, 0);
r1.setEnd(textNode, 2);
let r2 = new Range();
r2.setStart(textNode, 1);
r2.setEnd(textNode, 2);
let h1 = new Highlight(r1);
let h2 = new Highlight(r2);
h1.priority = 1;
h2.priority = 2;
CSS.highlights.set("foo", h1);
CSS.highlights.set("bar", h2);
</script>
The code above will display the following styled text, note that "b" is affected by both highlights h1 and h2, whereas "a" is only affected by h1:
abc
In this example there’s an event listener set on click events that logs the custom highlights present at the point where the click was made. The following sequences are some examples of what will be printed to console after a click:
[ h1 ], if the user clicks on character "a".
[ h2, h1 ], if the user clicks on character "b", as h2 has higher priority than h1.
[], if the user clicks on character "c".
The method highlightsFromPoint is defined as part of the HighlightRegistry interface as follows:
partial interface HighlightRegistry {
sequence<Highlight> highlightsFromPoint(float x, float y, optional HighlightsFromPointOptions options = {});
};
dictionary HighlightsFromPointOptions {
sequence<ShadowRoot> shadowRoots = [];
};
The highlightsFromPoint(x, y, options) method must return the result of running these steps:
If any of the following are true, return the empty sequence:
Otherwise, return a sequence of custom highlights given by ordering the highlights contained in this HighlightRegistry in descending order of priority, including only those highlights that contain at least one AbstractRange abstractRange that satisfies the following:
Let range be a Range object whose start node and end node are set to abstractRange’s start node and end node respectively, and start offset and end offset are set to abstractRange’s start offset and end offset respectively.
The coordinates x,y fall inside at least one of the DOMRects returned by calling getClientRects() on range.
Note: The specifics of hit testing are out of scope of this specification and therefore the exact details of highlightsFromPoint() are therefore too. Hit testing will hopefully be defined in a future revision of CSS or HTML.
The range’s commonAncestorContainer is not in a shadow tree or is in a shadow tree whose shadow root is contained by by options.shadowRoots.
Section on Events TBD, based on https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/master/highlight/events-explainer.md
should custom highlights have a dedicated event handling mechanism, or should that be added to pseudo-elements in general?
Appendix A. Privacy ConsiderationsThis section is non-normative.
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.
Appendix B. Security ConsiderationsThis section is non-normative.
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.
Appendix C. AcknowledgementsThis section is non-normative.
Acknowledge people (other than editors) who deserve credit for this.
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