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-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 (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
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The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.