Ned Freed writes:
Since it does mean that in email, I fail to see why such a change would be (a) Correct or (b) A good idea.
If «content-type: text/plain; charset=us-ascii", «content-type; text/plain» and a missing content-type field are equivalent, I would say that encoding the character set is mandatory in email, just as Jakob Palme wrote in the first message. A message sender can't avoid specifying the character set, even by leaving out the charset parameter.
It is quite true that a user of text/plain in email has no choice but to specify the charset, either implicitly by omitting the parameter, in which case it defaults to US-ASCII, or explicitly by including it. But that's not what Jacob said. He said:
The charset parameter is mandatory in the MIME content-type attribute.
This is quite clearly incorrect. The _parameter_ is not mandatory, _specification of the charset_ by one means or another is. (More precisely, it is unavoidable due to the way the defaults are set up. Referring to mechanisms involving defaults as mandatory is not good use of terminology IMO.) Now, perhaps Jacob meant to say that the charset of plain text has to be specified in some way. I'm not a mind reader and cannot divine intent. All I can do is respond to what he did say, which was incorrect. Ned