My problem is rather complex to explain, so I'll show you an example: http://ewolf.bplaced.de/misc/float.htm

I want to have a floated element (the blue box) to be be placed over two other elements (red and green) and I want the whole thing to be fixed-width and centered (done by the box with the black border) while the background of the red and green box should fill the whole width.

I'm actually not quite sure if the way I've done it now is XHTML/CSS valid, but it works - at least in Firefox. In IE6, the green box expands to fit the whole blue box - how can I fix this in IE6 or find another solution to show it correctly in all browsers?