I like that you can overwrite the var(headersize) in a different style. That makes these pretty powerful. However, var(headersize) is a lot to type, compared to @headersize (like LESS) or $headersize (like SASS). CSS allows for a lot of shorthand (#AABBCC = #AAA, table tr td a ~= table td a, * {}). Would be nice if there was a shorthand for variables too since they'll be used so much.
I think they don't do this because of consistency with other declarations like url() and the sort. CSS needs to be fast to process and easy to understand.
