This is a nice little resource/essay that outputs typographic css for your web design.
“The mechanics of the em unit offer an excellent way to size child elements in relation to their parents. In fact, if every child element defines its sizing values in em, a chain reaction is set off. Each child becomes proportionally bound to its parent, which in turn is bound to its parent, all the way up to the root element, ancestor of all. In this way, the proportions of the whole document end up being defined in relation to a single, shared value: the font-size of the body.
Documents sized in this way enjoy a golden property, one that most web pages would do well to provide: proportional scaling. Should the user or designer change the base font-size, all the other elements on the page will resize accordingly, preserving their original proportion to the body. It will look as if the view has just been zoomed in or out. “