:host() CSS pseudo-class
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since January 2020.
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. :host() has no effect when used outside a shadow DOM.
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:
While other functional pseudo-classes such as :is() and :not() accept a list of selectors as their parameters, :host() takes a single compound selector as its parameter. In addition, while :is() and :not() only take into account the specificity of their argument, the specificity of :host() is both the specificity of the pseudo-class and the specificity of its argument.
Try it
/* Following CSS is being applied inside the shadow DOM. */
:host(h1) {
color: red;
}
:host(#shadow-dom-host) {
border: 2px dashed blue;
}
<!-- elements outside shadow dom -->
<div id="container">
<h1 id="shadow-dom-host"></h1>
</div>
const shadowDom = init();
// add a <span> element in the shadow DOM
const span = document.createElement("span");
span.textContent = "Inside shadow DOM";
shadowDom.appendChild(span);
// attach shadow DOM to the #shadow-dom-host element
function init() {
const host = document.getElementById("shadow-dom-host");
const shadowDom = host.attachShadow({ mode: "open" });
const cssTab = document.querySelector("#css-output");
const shadowStyle = document.createElement("style");
shadowStyle.textContent = cssTab.textContent;
shadowDom.appendChild(shadowStyle);
cssTab.addEventListener("change", () => {
shadowStyle.textContent = cssTab.textContent;
});
return shadowDom;
}
/* 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 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: rgb(0 0 0 / 10%); 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 Shadow Module Level 1 # host-selector |
Browser compatibility
| desktop | mobile | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
:host()
|
|
|
|
|
|
|
|
|
|
|
|
|
Legend
Tip: you can click/tap on a cell for more information.
- Full support
- Full support
- See implementation notes.