They don't let you specify it because css transitions and animations are supposed to be simple and easy to use (no math), and css doesn't really have the langua

They don't let you specify it because css transitions and animations are supposed to be simple and easy to use (no math), and css doesn't really have the language constructs you'd need to write specify M(t). You can do it with javascript, which imho is far superior to animating with css.