it's not a css mistake, per se, but I always found difficult to reason about a way to tell "I don't really care how big this widget is, I care that this widget should be a rectangle with sizes of ratio X"; I want to be able to make a container, put a square on the top left, a 16/9 rectangle on the center and so on...
Yeah, setting an explicit aspect-ratio is on the roadmap. The relevant people (me and fantasai) have too many other layout-related things on our plate atm to deal with it, but it'll show up in the next year or two.
