[html] 100% height minus header?

As mentioned in the comments height:100% relies on the height of the parent container being explicitly defined. One way to achieve what you want is to use absolute/relative positioning, and specifying the left/right/top/bottom properties to "stretch" the content out to fill the available space. I have implemented what I gather you want to achieve in jsfiddle. Try resizing the Result window and you will see the content resizes automatically.

The limitation of this approach in your case is that you have to specify an explicit margin-top on the parent container to offset its contents down to make room for the header content. You can make it dynamic if you throw in javascript though.