yooouuuuu padding + margin + borders. In other words how to adjust your box-model to include ‘em. But you already knew that didn’t didn’t you.
box-sizing: border-box;/* or content-box or inherit */
So how about support:
Chrome (any): box-sizing
Opera 8.5+: box-sizing
Firefox : -moz-box-sizing (unprefixed since ??)
Safari 3: -webkit-box-sizing (unprefixed in 5.1+ versions)
And then there’s padding-box which is just weird (inset/outset on borders!..) and FF only so you can forget that. And, one more thing, there’s the min/max-height thingy. But you can checkout this excellent jsFiddle on that.