Variable

CSS variables defined specific values to be reused throughout a document.

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Class	Declarations
`$key:value`

Let's say this's your custom element and its tree:

<test-element>
    ▼ #shadow-root (open)
        <button part="button"></button>
</test-element>

and the element contains an encapsulated style like this:

:host {
    --button-bg: red;
}
 
[part=button] {
    background: var(--button-bg);
}

Elements in the shadow DOM cannot be selected via descendant selectors, but you can:

<test-element class="$button-bg:red $button-bg:pink@sm">
    ▼ #shadow-root (open)
        <button part="button"></button>
</test-element>

Conditionally apply

Apply styles in different states using selectors, media queries, etc.

<div class="$size:2.5rem:hover $size:2.5rem@sm $size:2.5rem@dark $size:2.5rem@print">…</div>