Whilst I can't see many people using multi-valued types client side.

> * q-values are specific to HTTP and thus not suitable for generic use.

What is specific to HTTP is the ability to provide extra information to
select a specific media type.  Even without HTTP, a browser could 
combine local and remote q values (not many browsers support local
q values) to choose a best match media type; it just doesn't have
the ability to select the appropriate one.  One could, however, envisage
a system where there were compound files from which a version could be
chosen.

> advisory type should be.  What if a resource is not of the type it
> says it is either as result of the type attribute or the Content-Type
> given in the response if HTTP is being used to get the resource?

HTML is declarative.  Unless there is an overriding policy issue
(e.g. that behind rejecting not-well-formed documents), the specification
shouldn't make normative requirements on behaviour, although it can
say MUST NOT or SHOULD NOT with regard to constructing HTML or configuring
servers so as to create this situation.