> In worst case complexity of styles resolution is O(nS*nD) where nS is a number of style rules and nD is a number of DOM elements. That's for current flat styl

> In worst case complexity of styles resolution is O(nS*nD) where nS is a number of style rules and nD is a number of DOM elements. That's for current flat style table used by CSS. You can optimize bits and cases but it in general problem is like that. Worst-case isn't really interesting for browsers, though. CSS doesn't have an adversarial performance model. > @media and @set are different mechanisms. :root in @set matches element that has this set applied. @set's are scoped set of rules. @media are not. Again, this roughly matches @scope (where :scope matches the element that has the given selector applied). But this is just a fantasy spec, right? There are no browsers that implement this, and just like with @scope or nesting or CSS at all, there's no guarantee in the spec what performance characteristics you would have? FWIW, if you want something like this and get it through CSSWG, you'll most likely see it end up with worse performance than flat CSS, unless it becomes _really_ popular and browsers see the need to optimize certain sub-cases of it (the general case, with tons of different sets, will be nearly impossible to make really fast).