As a twelve year Internet veteran, let me first correct your terminology:
all "browsers" read HTML; that's their job. Some of them also read mail,
which isn't their job, but many people think it is. Email has been
around for a lot longer than the web has, and has developed its own
standards over time to allow interchange between the millions of
systems supporting it, hundreds of thousands of which have never even
heard of HTML. Email programs support lots of features that have
developed over those years to make email more powerful and useful.
Many browsers fail to support many of those features, and instead like
to make up their own standards outside of the Internet community so
they can brag about "features" that have already been available in
standardized forms.
HTML is not and has never been a valid format for email. It has
never been endorsed by any standards organization, it is not supported
by most serious email programs, and is a serious waste of bandwidth
and nuisance to those of us who use real email programs. The MIME
format "text/enriched" /is/ a valid format, and less intrusive, and
understood by real email programs (but not, alas, by the idiots who
write Netscape mail and MS Exchange, so it should still be avoided).
All communication requires standards. Without them, one would have
to tailor every communication to its specific audience, which is the
impossible state of affairs the standards were created to avoid.
Where the standards fail, expanding them is justified. But where they
work well, and have worked well for years, flagrantly violating them
is pointless and rude.
P.S. Thanks for the opportunity to expand ny Web page's rant about
standards. The email nonsense hadn't occurred to me at the time. :)