You mean use MIME content types for HTTP2 format negociation: exactly!
> Would MIME in return (a) add
>text/SGML/HTML as a MIME format
Just write a spec for it and register it with the IANA.
> (who registers DTD names?
The ISO grants public text identifiers for DTDs.
> and also
>use an IETF standard URL a the external reference format?
I'm not sure I understand this part. Sure you can have a format
called text/html, and sure you can put URLs in it, but that
doesn't mean URLs have to become part of the MIME standard.