>What does this do for senders that only include the HTML part with no
 >plain text part?

It allows the HTML code to go through.  What is annoying is that there is no
word wrap enforced and so you have to scroll (and scroll and scroll) the
browser if you want to read it.

I would still advocate some handling of "printed quotable" format, as
non-techinical people, particularly on support lists, don't want to hear how
they have to reconfigure their mailer for plain text, and since the masses
aren't from the olden days, don't care about historical accuracy as to why that
should be the default setting in spite of nobody doing it anymore.

Randy