[frivoal]

> 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.

[_bxg1]

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.