I don’t know that I’d call this an implementation detail of the engine.

As I read this, the spec would force the implementation on every engine - and there is no known algorithm that could answer the question in constant time given the spec.

Infinite look ahead means infinite look ahead. Which means (IIUC), every time this case is encountered, any/every CSS engine (existing today or yet to be built - every CSS engine from this point forward) would have to potentially check every subsequent token until it could properly understand the context of the symbol.

Given that - I don’t like the idea of any CSS file being able to put my browser into that state.