However, this approach might have other benefits - 
Allowing Content-Type: multipart to be returned could allow:
A way to generically inline images and other objects without extending
HTML with more <IMG> like tags for other types. If the type is not 
understood, a marker should be displayed by the browser. 
( Problem: how not to send things that the browser doesn't understand. 
  If each part has to be parsed and type-checked, this might eliminate
  of the advantage of using multipart. ) 
[ With browser support, this could allow sending audio parts 1,2,3,4, 
etc. and start playing part 1 while the rest are being transferred, 
automatically playing them in sequence. ] 
text/html and text/plain could be alternated, allowing a simple 
way of inserting text without having to filter and escape characters 
first. 
Allow multipart/parallel audio|image and video. Semantics are that
they are intended to be presented together. ( within the "best effort"
of the client. ) 
[ Functionality of multipart/alternative is already in the client-server
  negotiation, so support would be redundant. ] 
All of which would, I think, take minimal effort on the server to
implement, but WOULD take a substantial effort for clients to support. 
- Steve Majewski       (804-982-0831)      <sdm7g@Virginia.EDU>
- UVA Department of Molecular Physiology and Biological Physics