CODEWITHSUNDEEP
cssweb-componentshadow-dom
Philipp Gfeller
Philipp Gfeller

Reputation: 1259

Can the CSS :part pseudo-selector be used to style nested web components?

When creating Web Components with encapsulated styles using Shadow DOM, parts of the shadowed markup can be styled using the ::part pseudo selector (https://developer.mozilla.org/en-US/docs/Web/CSS/::part).

Can the part selector be used to target nested shadow parts?

<custom-element>
  #shadow-root
    <div part="thats-easy"></div>
    <another-custom-element part="proxy-part">
      #shadow-root
        <div part="target-me"></div>
    </another-custom-element>
</custom-element>

Current efforts were fruitless:

another-custom-element::part(target-me) { }
custom-element::part(proxy-part) another-custom-element::part(target-me) { }
custom-element::part(proxy-part another-custom-element::part(target-me)) { }
custom-element::part(proxy-part::part(target-me)) { }
```

Upvotes: 7

Views: 5030

Answers (2)

luwes
luwes

Reputation: 884

Fun twist, it is possible and TIL about exportparts
https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/exportparts

Example 1

<style>
  mux-player::part(textspan) { color: red; }
</style>

<mux-player>
  <template shadowrootmode="open">
    <media-poster-image exportparts="poster, img, innerspan: textspan">
      <template shadowrootmode="open">
        <img part="poster img" src="...">

        <span part="innerspan">
          This text will be red because the containing shadow host forwards innerspan to the document as "textspan" and the document style matches it.
        </span>

        <span part="textspan">
          This text will not be red because textspan in the document style cannot match against the part inside the inner custom element if it is not forwarded.
        </span>
      </template>
  </template>
</mux-player>

Example 2

Example in production with multiple nested custom elements with CSS parts.
https://github.com/muxinc/elements/blob/main/packages/mux-player/src/template.ts

Upvotes: 7

Harshal Patil
Harshal Patil

Reputation: 20970

Nope. It is not possible. It kind a breaks the encapsulation principle. The right way is to use proper theming. That means using a combination of:

::part - For direct theming of the component
:host-context - for theming based on the context
::slotted - For styling the slotted element inside the styling

For more dynamic theming, you can use above styles in combination with Element.matches API. Whatever the class/context that the user of the component has set, you can then change the styling of nested children component.

On a side note, modifying the styling of a decadent component (children of children) is a bad practice even when not using Shadow DOM or Web Components. It will result in a brittle non-maintainable CSS.

Edit Note:

:host-context is not implemented in all browsers and probably never will.

Upvotes: 3

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