The amount of lookahead necessary dictates what algorithms you can use for parsing. If you go from 1-token lookahead to unbounded lookahead, you might force a l

The amount of lookahead necessary dictates what algorithms you can use for parsing. If you go from 1-token lookahead to unbounded lookahead, you might force a lot of people to rewrite their parsers or make major changes (because they would have to use different algorithms). This would mean that a lot of CSS parser would simply not adopt the new syntax, at least not in a timely fashion. If you keep 1-token lookahead, existing CSS parsers can just drop this in.