I guess the idea would be something along the lines of a 'strict' tag, to identify a shift to modern syntax with stricter rules around parsing, that ensures an infinite lookahead wouldn't be required for e.g. pseudos vs properties.
I put together a regex last time this was discussed [1] (which obviously completely alleviates all concerns): https://regex101.com/r/aOc2Pz/1
But seriously, syntax evolutions happen. If there's an issue with parsing garbage CSS why not just let that garbage run slowly, and re-optimise for the rest, prioritising for form and function.