oppsss winblows and netscape have that [send HTML] as default! sorry. but then again this IS the 20th century, you could get a mail reader that knows what html is. or you still use mail on the command line?
Oops - it's the 21st! :-)
You might consider accepting the request to send in plain email, since first of all it's a matter of etiquette, not of what's possible. Also, some of us receive the list in digest form, that is, only text mode. The HTML overhead shows up in that mode as worthless gobbledegook. People who post in HTML or quote the entire digest basically get ignored.
Sure, one could go through the extra work (and horrid security risk, if received directly) to read messages (rudely) posted in HTML, but why should one? If a person is too arrogant to follow the simple rules of courtesy politely requested by the community, then his opinions are probably not going to be missed.
Your call: arrogant or polite?