Re: [csswg-drafts] [css-mixin-1] How does Shadow DOM influence mixin lookup? (#12671)

>  it allegedly defeats @sorvell's primary use case

That's correct. Strict matching would mean ~@mixin has the potential to largely address the flexibility issues with styling Shadow DOM~.

> Additionally, [tree-scoped names](https://drafts.csswg.org/css-shadow-1/#css-tree-scoped-name) can be loosely matched or strictly matched, defaulting to [loosely matched](https://drafts.csswg.org/css-shadow-1/#tree-scoped-name-loosely-matched) unless otherwise specified.

Given that loose matching is the default, @emilio what is the justification for matching strictly?

Making @mixin loosely matched conceptually aligns them with custom properties and overcomes some of the limitations with custom properties that led to the introduction of parts described in the[ motivation section here](https://drafts.csswg.org/css-shadow-1/#motivation). Unfortunately, parts are cumbersome due to the limitations of `exportparts` which [failed to be separately addressed](https://github.com/WICG/webcomponents/issues/1051).

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Received on Thursday, 26 March 2026 12:49:29 UTC