:host-context()

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.

In other words, this allows a custom element, or anything within that custom element's shadow DOM, to apply different styles based on its position within the outer DOM or classes/attributes applied to ancestor elements.

One 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>.  Another typical use would be to allow inner elements to react to classes or attributes on any anscestor elements - for example, applying a different text color when a .dark-theme class is applied to <body>.

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;
}

/* Changes paragraph text color from black to white when 
   a .dark-theme class is applied to the document body */
p {
  color: #000;
}

:host-context(body.dark-theme) p {
  color: #fff;
}

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

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
:host-context()Chrome Full support 54Edge Full support 79Firefox No support No
Notes
No support No
Notes
Notes See bug 1082060.
IE No support NoOpera Full support 41Safari No support NoWebView Android Full support 54Chrome Android Full support 54Firefox Android No support No
Notes
No support No
Notes
Notes See bug 1082060.
Opera Android Full support 41Safari iOS No support NoSamsung Internet Android Full support 6.0

Legend

Full support  
Full support
No support  
No support
See implementation notes.
See implementation notes.

See also