Originally Posted by qim
I tested the snippet in EditPlus and it gave me a green sidebar on the left, a right sidebar on the right; and a blue area below the green sidebar, rather than in the center.
I don't know what EditPlus is but it is not a browser. You should test your markup the same way it's going to be used in the real world, in a browser. That's the reason for the suggestion in my signature line.