Seems like a premature optimization at the cost of the user.

I don't care how fast it runs if it doesn't do what I want.

EDIT: yeah, you probably are right ... I know too little about parsers. Maybe there exists a formal proof that you can't make a fast parser for that syntax without unbounded lookahead.

Maybe they should even create CSS from the start as two part system. Convenient syntax for humans, once written compiled to StyleAssembly comfortable for the browser.