:host()

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; }

css

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

Syntax

css

: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:

html

<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 Scoping Module Level 1 # host-selector