Unsere Freiwilligen haben diesen Artikel noch nicht in Deutsch übersetzt. Machen Sie mit und helfen Sie, das zu erledigen!
Sie können den Artikel auch auf English (US) lesen.
This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
The :host-context()
CSS pseudo-class function selects the shadow host of the shadow DOM containing the CSS it is used inside (so you can select a custom element from inside its shadow DOM) — but only if the selector given as the function's parameter matches the shadow host's ancestor(s) in the place it sits inside the DOM hierarchy.
A typical use of this is with a descendant selector expression — for example h1
— to select only instances of the custom element that are inside an <h1>
.
Note: This has no effect when used outside a shadow DOM.
/* Selects a shadow root host, only if it is a descendant of the selector argument given */ :host-context(h1) { font-weight: bold; } :host-context(main article) { font-weight: bold; }
Syntax
:host-context( <compound-selector-list> )where
<compound-selector-list> = <compound-selector>#
where
<compound-selector> = [ <type-selector>? <subclass-selector>* [ <pseudo-element-selector> <pseudo-class-selector>* ]* ]!
where
<type-selector> = <wq-name> | <ns-prefix>? '*'
<subclass-selector> = <id-selector> | <class-selector> | <attribute-selector> | <pseudo-class-selector>
<pseudo-element-selector> = ':' <pseudo-class-selector>
<pseudo-class-selector> = ':' <ident-token> | ':' <function-token> <any-value> ')'where
<wq-name> = <ns-prefix>? <ident-token>
<ns-prefix> = [ <ident-token> | '*' ]? |
<id-selector> = <hash-token>
<class-selector> = '.' <ident-token>
<attribute-selector> = '[' <wq-name> ']' | '[' <wq-name> <attr-matcher> [ <string-token> | <ident-token> ] <attr-modifier>? ']'where
<attr-matcher> = [ '~' | | | '^' | '$' | '*' ]? '='
<attr-modifier> = i | s
Examples
The following snippets are taken from our host-selectors example (see it live also).
In this example we have a simple custom element — <context-span>
— that you can wrap around text:
<h1>Host selectors <a href="#"><context-span>example</context-span></a></h1>
Inside the element's constructor, we create style
and span
elements, fill the span
with the content of the custom element, and fill the style
element with some CSS rules:
let style = document.createElement('style'); let span = document.createElement('span'); span.textContent = this.textContent; const shadowRoot = this.attachShadow({mode: 'open'}); shadowRoot.appendChild(style); shadowRoot.appendChild(span); style.textContent = 'span:hover { text-decoration: underline; }' + ':host-context(h1) { font-style: italic; }' + ':host-context(h1):after { content: " - no links in headers!" }' + ':host-context(article, aside) { color: gray; }' + ':host(.footer) { color : red; }' + ':host { background: rgba(0,0,0,0.1); padding: 2px 5px; }';
The :host-context(h1) { font-style: italic; }
and :host-context(h1):after { content: " - no links in headers!" }
rules style the instance of the <context-span>
element (the shadow host in this instance) inside the <h1>
. We've used it to make it clear that the custom element shouldn't appear inside the <h1>
in our design.
Specifications
Specification | Status | Comment |
---|---|---|
CSS Scoping Module Level 1 The definition of ':host-context()' in that specification. |
Working Draft | Initial definition. |
Browser compatibility
The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.
Desktop | Mobile | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Basic support | Chrome Full support Yes | Edge No support No | Firefox
No support
No
| IE No support No | Opera Full support Yes | Safari No support No | WebView Android Full support Yes | Chrome Android Full support Yes | Edge Mobile No support No | Firefox Android
No support
No
| Opera Android Full support Yes | Safari iOS No support No | Samsung Internet Android Full support Yes |
Legend
- Full support
- Full support
- No support
- No support
- Experimental. Expect behavior to change in the future.
- Experimental. Expect behavior to change in the future.
- See implementation notes.
- See implementation notes.