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: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 selects the shadow host of the shadow DOM containing the CSS it is used inside — in other words, this allows you to select a custom element from inside its shadow DOM.
Note:
This has no effect when used outside a shadow DOM.
/* This CSS is being applied inside the shadow DOM. */
:host {
background-color: aqua;
}
<h1 id="shadow-dom-host"></h1>
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 */
:host {
font-weight: bold;
}
The following snippets are taken from our host-selectors example (see it live also).
In this example we have a basic 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:
js
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 { background: rgb(0 0 0 / 10%); padding: 2px 5px; } rule styles all instances of the <context-span> element (the shadow host in this instance) in the document.