Here, each span was placed in its corner using simple absolute positioning (-1px values to cover the border). In this method, padding is needed at all sides of the box, but what's nice is it can be in relative units (1em here).
Using the selector * html body, I gave IE's container a width of 95% then reset it to inherit for browsers that understand child selectors, notably IE5/mac.
Success: Mozilla, Opera 7, IE6 (w/ annoying rounding errors)