I learned something the other day when we were fleshing out the background properties in the Almanac. It's such an intuitive thing, that I guess I just never stopped to think about it before.
By default, background-position is
0 0, the
top left corner. Now imagine you set it to
50% 0. That background image is now center top. It figures out the size of that image, and positions the center of it at the center of …