To elaborate why you need unlimited lookahead, consider the following:

    h1:has(+ p) { something: anything; } h1 { something: nothing; }
    everything: has(+ p) { something: anything; }; h1 { something: nothing; }

Syntactically speaking these two lines are equivalent except for a single semicolon; the differing first token doesn't change anything about that. And you can absolutely add a custom element that looks like a property name to blur a line further. So in order to see if the first block is a rule or a property, you absolutely have to read up to the first `;`, excluding nested braces and alikes.

That said, the main problem is that the property syntax is overly forgiving, not that the nested rule syntax is problematic. Something akin to JS `use strict` should be able to restrict the property syntax enough to allow the nested rule syntax without unlimited lookahead.