:host-context()
Quick Summary for :host-context()
: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.
Code Usage for :host-context()
/* 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; } More Details for :host-context()
: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 # host-selector |
See also
Web components:host :host() Select your preferred language English (US)Français日本語中文 (简体) Change language
