"But I don't want to put images in my markup!" Fine, then if you still want to achieve the 100%-fluid width boxes, you need to set the height of the .box to match its width with some simple jQuery. And be sure to fire it whenever you resize the window.
Don't want to use any JS? Use images (if it makes sense (if I catch you using transparent pngs to achieve this, I'm going to be very angry!)).