Well, as a person who uses DIVs for everything (i fkn hate tables and all that, haha) I'll offer some advice.
I put all my general CSS (body etc) in a external file.
Then, I put all the CSS for the pages DIVs in the head tag of the page.
For some reason I find this easier if the DIVs tend to need the same postions (mainly) and maybe a few alterations from page to page, just copy, paste and change.
If all your DIVs are going to be the same for each page.
Just put them in the external file.
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If you're not actualy naming DIVs, and using the geric DIV tag.
Don't bother with css for DIV. Just use something like:
<div height="XXpx" width="XXpx"></div>
and so on.
hope that helps
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Also, for XML.
www.w3schools.com will teach you what you need to know.
But considering your posts main topic, I think you mean XHTML.