Lookahead is how many tokens the parser needs to see before it can make a decision what branch it is in and start ingesting those tokens properly. In `& color:

Lookahead is how many tokens the parser needs to see before it can make a decision what branch it is in and start ingesting those tokens properly. In `& color:hover` you've got at least four tokens (making some basic assumptions about how the tokenizer works and ignoring whitespace): 1 & 2 color 3 : 4 hover In this case you've got that `&` in the first token and the parser knows what to do with that (only appears in nesting scopes). (No lookahead needed.) In the case of `color:hover &` the situation reverses: 1 color 2 : 3 hover 4 & Tokens 1/2/3 are ambiguous (could be a property:value or could be tag selector:state) and it isn't until token 4 that it becomes obvious that it must be a nested selector because that & token is the disambiguator. So here you need a lookahead of 4 tokens just to disambiguate. The problem is "unbounded" because the way selectors and values work they can be arbitrarily long. You can have "long property values". This doesn't make a lot of sense in current CSS, but is a valid property value assignment: color:hover, tt, .frankenstein, p; This is a possible (weird) nesting: color:hover, tt, .frankenstein, p & { } The number of tokens needed to lookahead is "unbounded" because you can keep adding tokens to that weird property value or to the nesting example with no clear limit.