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And then there's another layer of complication when you assume that any and all styles can change at any time. A big part of "contain"s use-case is when you can guarantee that your JS will never apply certain styles to certain elements. There's no way the browser could ever verify that.

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> when you can guarantee that your JS will never apply certain styles to certain elements

It goes further than that: using contain means that even if you tried applying these styles that would break optimizations, the browsers will not obey you. css-contain isn't just a hint to the browser that you're not screwing things up, it's a mode switch that prevents you from doing so.

True, but the things it prevents you from doing are virtually never something you would want to prevent via a mechanism like this. You'd want to catch them higher up. I don't think it would really be helpful as a constraints system.

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