In article <1149239813.134006.175360@i...>,
 "Andy Dingley <dingbat@c...>" <dingbat@c...> 
 wrote:

> My question is, is there any way to author a non-HTML DTD (SGML or XML)
> so as to restrict valid documents to only allow a certain subset of
> their elements to be used as the root element? Can this restriction be
> expressed _entirely_ within a DTD?

No and no. 

RELAX NG can restrict the allowed roots and does not allow the document 
to override. 

> Is this fragment a valid HTML document ?

> <!DOCTYPE div PUBLIC "-//W3C//DTD HTML 4.01//EN"
> "http://www.w3.org/TR/html4/strict.dtd">
> <div>
>     <p>Foo</p>
> </div>

Valid in the SGML sense but not conforming to the HTML 4.01 spec. 
Validity is overrated. DTD-validity is especially overrated.

> Good tutorial refs on DTDs are also welcome. I don't know anything like
> enough on DTD innards.

Since you haven't learning invested in DTDs, unless you have a 
non-negotiable requirement to use them, I suggest learning RELAX NG 
Compact Syntax instead:
http://relaxng.org/compact-tutorial-20030326.html

-- 
Henri Sivonen
hsivonen@i...
http://hsivonen.iki.fi/
Validation Service for RELAX NG: http://hsivonen.iki.fi/validator/