:host()

The :host() 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.

The most obvious use of this is to put a class name only on certain custom element instances, and then include the relevant class selector as the function argument. You can't use this with a descendant selector expression to select only instances of the custom element that are inside a particular ancestor. That's the job of :host-context().

Note: This has no effect when used outside a shadow DOM.

/* Selects a shadow root host, only if it is
   matched by the selector argument */
:host(.special-custom-element) {
  font-weight: bold;
}

Syntax

:host( <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(.footer) { color : red; } rule styles all instances of the <context-span> element (the shadow host in this instance) in the document that have the footer class set on them — we've used it to give instances of the element inside the <footer> a special color.

Specifications

Specification
CSS Scoping Module Level 1
# host-selector

Browser compatibility

BCD tables only load in the browser

See also