Ah, unsetting. I get what you’re pointing at there, but my immediate reaction is surprise that this is relevant for your approach, which seems to be all about avoiding situations where that has an effect in the first place. Utility styles and explicitly shunning the cascade.

revert-layer <https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/V...> may be relevant to the situation you describe. Not the simplest thing, but it’s a tool most don’t know about yet which can do that kind of thing.

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On :where(). It’s easy to see why :has() can tank performance: it’s doing a whole new complicated thing. :where() is a good deal simpler, and this specific form of it (where it’s the entire selector) has only the effect of zeroing specificity. I’d be mildly surprised if the performance difference from wrapping an entire selector in :where(…) was measurable even in a worst-case synthetic document.