XML Schema and DocBook

Yuhůů points to a slide presentation on 
 XML Schema. As he rightly says,
 it replaces, or more precisely extends, DTD, which couldn't express
 more complex relationships. I'll add that XML Schema is one of the many 
 standards maintained by the W3C. See XML schema.

There's another interesting point at the end of Yuhůů's post.

Incidentally, the slides from Jirka Kosek's talks are a lovely example of what you can do with HTML. And with XML, too. I'm sure he originally wrote it all in XML, probably in Emacs, and exported it to HTML with XSLT.

The presentation was made with DocBook, and yes, it really is XML
 (DocBook itself is just a DTD).
 The transformation uses XSLT and can produce
 HTML, XHTML, PDF, RTF (Word), CHM (HTML Help), PostScript, JavaHelp, and any number of other formats. It just depends 
 on which XSLT stylesheets you use.

DocBook was originally intended for writing documentation, but I'll leave the rest to someone who knows it better than I do.
 Take a look at Jiří Kosek's 
 
 DocBook, a brief introduction to creating and processing documents
 . I'm not sure what you'd call brief, but there's enough information there for a book of its own.
 One final point. Jiří Kosek is highly respected in
 the DocBook community for his contributions, including his work on 
 the transformation stylesheets.