Do you want to know how the CSS WG works? Fantasai has written about:csswg, An Inside View of the CSS Working Group at W3C.
url()syntax definition to match grammar, specifically, drop the requirement to escape commas since implementations don’t require it and existing pages depend on that.
:nth-child(). Comments and spaces allowed at other (token) boundaries.
//comments in CSS—nice to have, but too late to add. See follow-up comments on mailing list for why introducing this would be actively harmful.
margin-topis always the top margin even in other writing-modes.