HTML in MIME mail

Hi, what is the current state of the art in sending HTML body
parts in a MIME mail message?  I have heard of some non-standard
attempts at using "Content-type: text/charset=HTML".  I guess that
the only really legal approach in the short term would be something
like: "Content-type: application/XHTML"?

Providing HTML body parts would make mail really powerfull.  You could
send a friend an entire Web page.  This surely beats external body
parts in functionality and ease of use.

-Mark Joseph