[spankalee]

You're right. If we're talking about only the root element, it is similar to :host, because the custom element is itself stylable from the outside.

What makes me say that encapsulation is weakened is that threading JS property bags to other internal elements is also relatively common, and inline styles don't really offer any encapsulation there. Other selectors in the page can still select and style properties that are not directly styled by the element. I should have clarified that.

Shadow DOM provides true runtime style encapsulation, and the selectors on the page can't select the internal elements at all.

[ss3000]

Ah I see, that makes sense. Shadow DOM definitely offers much stronger style encapsulation than a React component where you can only "enforce" that components shouldn't reach into it to style internals by convention.