lost in namespaces (and invalid Atom feed)

Hello,

I am trying to produce a simple (mainly) XSLT-based system for
producing Atom feed (to be published on wordpress.com, but I try
to be as platform-independent as possible) from Docbook. Current
state of the project is on
http://mcepl.fedorapeople.org/tmp/atom-wordpress/

However, I am confused by http://tinyurl.com/cqw2p8 ... 

How come

<div xmlns="http://www.w3.org/1999/xhtml">

is undefined content element, when it is in well defined
namespace?

Could somebody suggest to me some articles on XSLT, Atom and
namespaces ... all I could find on the xml.com and similar places
seems to me too simple?

Thank you very much for any hints.

Matěj Cepl

P.S.: sorry, for slightly off-topic post to docbook NG, but this 
is part of my project how to write blog in Docbook, so hopefully 
at least somebody might be interested.