Наши волонтёры ещё не перевели данную статью на Русский. Присоединяйтесь к нам и помогите сделать эту работу!
Вы можете также прочитать эту статью на English (US).

This is an experimental technology
Check the Browser compatibility table carefully before using this in production.

The :host() 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.

The most obvious use of this is to put a class name only on certain custom element instances, and then include the relevant class selector as the function argument. You can't use this with a descendant selector expression to select only instances of the custom element that are inside a particular ancestor. That's the job of :host-context().

Note: This has no effect when used outside a shadow DOM.

/* Selects a shadow root host, only if it is
   matched by the selector argument */ 
:host(.special-custom-element) {
  font-weight: bold;
}

Syntax

:host( <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

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(.footer) { color : red; } rule styles all instances of the <context-span> element (the shadow host in this instance) in the document that have the footer class set on them — we've used it to give instances of the element inside the <footer> a special color.

Specifications

Specification Status Comment
CSS Scoping Module Level 1
The definition of ':host()' in that specification.
Working Draft Initial definition.

Browser compatibility

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support Yes No1612 3 No Yes No
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
Basic support Yes Yes No1612 3 Yes No Yes

1. Under consideration

2. See bug 1205323

3. From version 61: this feature is behind the dom.webcomponents.shadowdom.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.

See also

Метки документа и участники

Внесли вклад в эту страницу: mfluehr, jpmedley, jaller94, chrisdavidmills
Обновлялась последний раз: mfluehr,