Web/CSS/ host()

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 | s

Examples

Selectively styling shadow hosts

The following snippets are taken from our host-selectors example ([[../../../../../../mdn.github.io/web-components-examples/host-selectors/index|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 1The definition of ':host()' in that specification.	Working Draft	Initial definition.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
`:host()`	Chrome Full support 54	Edge Full support 79	Firefox Full support 63 Full support 63 No support 61 — 63 Disabled' Disabled' From version 61 until version 63 (exclusive): this feature is behind the `dom.webcomponents.shadowdom.enabled` preference (needs to be set to `true`). To change preferences in Firefox, visit about:config.	IE No support No	Opera Full support 41	Safari Full support 10 Notes' Full support 10 Notes' Notes' Certain CSS selectors do not work (:host > .local-child) and styling slotted content (::slotted) is buggy.	WebView Android Full support 54	Chrome Android Full support 54	Firefox Android Full support 63 Full support 63 No support 61 — 63 Disabled' Disabled' From version 61 until version 63 (exclusive): this feature is behind the `dom.webcomponents.shadowdom.enabled` preference (needs to be set to `true`). To change preferences in Firefox, visit about:config.	Opera Android Full support 41	Safari iOS Full support 10 Notes' Full support 10 Notes' Notes' Certain CSS selectors do not work (:host > .local-child) and styling slotted content (::slotted) is buggy.	Samsung Internet Android Full support 6.0

Legend

Full support: Full support
No support: No support
See implementation notes.': See implementation notes.
User must explicitly enable this feature.': User must explicitly enable this feature.