Notes:
Workaround of empty cells is required to keep the borders continuous in IE. In complex structures it would become necessary even without IE, while on borderless structures they'd probably be waste. div.colspan>div>div{position:relative} is also required due to IE, so that inner wrapper-div in cell 3 goes over the border of it's cell. Double-wrapper is required since CSS-cells wont allow overflow, they stretch instead.

Edit:
From original design, I changed so that spanning cell and hidden cell is marked as borderless, and removed background-color from second wrapper. This is more printer-friendly, since printing by default leaves background-colors out.