XML Design

[JR]

I am trying to design a site using XML. I want to make it browser independant
so I will convert to HTML on the server.

Most pages contain information on different topics (i.e. multiple roots).
Is it proper design to have one DTD that contains all possible data items?
This would make each page have the same root element.

Any insight on how to really apply XML would help. Most examples just show
how to display employees using XSL and XML.

How do I display a list of departments on the left, the info on the selected
departments on top with a list of employees in the department?

Thanks in advanced.

JR