Just a small “nit-picky” detail – an “em” on the web is the height of the M, not the width. The em does refer to the width of the M in the print industry where it was the width of the block of metal used in old printing presses – as the M generally filled out the entire block. However, since we don’t have “metal blocks” in the digital world, we simply use the height of the M.

Also, IE7 introduced support for min-width/min-height: http://msdn.microsoft.com/en-us/library/cc351024(v=vs.85).aspx

Otherwise, great write-up!