Adding new Doctype

Hello Gerald, everybody,

Can somebody point me to a thread that helps to understand
what has to be done for adding a new document type? For example,
I would like to look into adding support for XHTML 1.1
http://www.w3.org/TR/2001/PR-xhtml11-20010406/ when/if
that makes it to Recommendation.

Regards,   Martin.