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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>) {
  /* ... */
}

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:

const style = document.createElement("style");
const 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

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`:host-context`	54	79	No See bug 1082060.	No	41	No	54	54	No See bug 1082060.	41	No	6.0