This is an experimental technology
Check the Browser compatibility table carefully before using this in production.
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.
A 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>
.
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; }
: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
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.
Specification | Status | Comment |
---|---|---|
CSS Scoping Module Level 1 The definition of ':host-context()' in that specification. | Working Draft | Initial definition. |
Desktop | ||||||
---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | |
Basic support | Yes | No | No | No | Yes | No |
Mobile | |||||||
---|---|---|---|---|---|---|---|
Android webview | Chrome for Android | Edge Mobile | Firefox for Android | Opera for Android | iOS Safari | Samsung Internet | |
Basic support | Yes | Yes | No | No | Yes | No | Yes |
© 2005–2018 Mozilla Developer Network and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/CSS/:host-context()