Tired of declaring the width of an element and then having to subtract from it as padding and borders are added later?

* { box-sizing:border-box; } is the set it and forget it of box models. Set your desired width and leave it intact as you later add padding and borders; they don’t affect the overall width.

Paul Irish has more of the specifics on his blog.

And in case you were wondering, margins are rendered the same either way.

(via Jeff Croft)