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

Browser compatibility