Add another Header row resulting in multiple header rows for GridView

It was quite easy to add rows to the header of the Gridview thanks to this article:
Multi Header Grid view - The Code Project - ASP.NET
It's not easy to understand the description, but if you download the sample code you can easily see how it is done.

TabContainer: Extra white space below tab title when removing the DOCTYPE

This forum thread helped me solve a rendering problem with the TabContainer when in a page without DOCTYPE.
TabContainer: Extra white space below tab text - ASP.NET Forums