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

Issue

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)) { }
```

Solution

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.

Answered By - Harshal Patil