The discussion suggests that the following be reworded in MIME(draft).
The syntax of "richtext" is very simple. It is assumed, at
the top-level, to be in the US-ASCII character set, unless
of course a different charset parameter was specified in the
Content-type field.
Here is a proposed revision, which reverses the order of it:
The syntax of "richtext" is very simple. It is assumed at
the top-level to be in a character set specified in a
Content-type charset parameter, unless there is no
Content-Type field, in which case it is assumed to be in the
US-ASCII character set,
Best...\Stef