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.

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

: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> )
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 (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
CSS Scoping Module Level 1 # host-selector

See also

Web components :host :host-context()

Last modified: Aug 12, 2021, by MDN contributors

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