To follow up on what Ed Levinson said ...
To state the problem: Multipart/Related requires a type
parameter which gives the type ot the object's root. If the root
is Multipart/Alternative, then the MUA doesn't know what the type
is until after the Alternative is resolved.
Maybe I'm being thick. I don't see a problem, yet. What is it
that the MUA _needs_ to know that Multipart/Alternative doesn't tell it?
Al Gilman