Output polyglot XHTML rather than HTML

No need to lose the benefits of XML when we know that the document can't
be malformed.  Output as HTML-compatible XHTML.

Note:  We're already using the XHTML doctype rather than the HTML one.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>
Use XHTML 1.0 Strict DOCTYPE

We don't need any of the features present in transitional XHTML, so
declare that we conform to strict.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>