well, yes. CSS has variables syntax too (http://dev.w3.org/csswg/css-variables/ http://dev.w3.org/csswg/css-variables/) but for the time of compiling you don't

well, yes. CSS has variables syntax too (http://dev.w3.org/csswg/css-variables/ http://dev.w3.org/csswg/css-variables/) but for the time of compiling you don't know anything about the runtime, so you can't calculate things like "10em + 5px" 'cause EM's absolute value depends on many things, so I had to separate "real" css calculations and "fake" ones