:host-context()
The :host-context()
CSS pseudo-class allows you to style elements within a shadow DOM differently based on the selector of the shadow host (the element that has the shadow root) and its DOM ancestors.
Normally, elements within a shadow DOM are isolated from the DOM outside of it. The :host-context()
allows you to "peek outside" of this Shadow DOM and check if any of the element's ancestor elements match a certain CSS selector. For example, applying a different text color to elements within a shadow root when a .dark-theme
class is applied to <body>
.
Think of it like this: Imagine you have a <greenhouse>
custom element, that has a <chameleon>
living inside. Here, the <greenhouse>
is the Shadow DOM host and the <chameleon>
element is within the Shadow DOM. The :host-context()
lets the <chameleon>
change its appearance based on the <greenhouse>
's environment. If the <greenhouse>
is in a sunny location (has a "sunny-theme" class), the <chameleon>
turns yellow. If the <greenhouse>
is in a shady spot (a "shady-theme" class applied instead), the <chameleon>
turns blue.
This selector pierces through all shadow boundaries. It will look for the sunny or shady theme applied directly to the <greenhouse>
or on any of the host's ancestors and ancestor DOMs all the way up until it reaches the document root.
To limit the selector to only the <greenhouse>
host directly or limit the selection to host's DOM, use the :host
or :host()
pseudo-class instead.
Note: This has no effect when used outside a shadow DOM.
The specificity of :host-context()
is that of a pseudo-class, plus the specificity of the selector passed as the function's argument.
Try it
/* Selects a shadow root host, only if it is
a descendant of the selector argument given */
:host-context(h1) {
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(.footer) { color : red; }" +
":host { background: rgb(0 0 0 / 10%); 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
BCD tables only load in the browser
See also
- Web components
- CSS
:host
pseudo-class - CSS
:host()
pseudo-class - CSS
:state()
pseudo-class - CSS
::slotted
pseudo-element - HTML
<template>
element - CSS scoping module