May I make a suggestion? if you're using Thunderbird as your mail application, you can send both HTML and plain text messages, that way you most likely reach both groups of users... I personally like HTML mail too, but for many people this is not the case, since they may use plain text console mail readers. Just a suggestion, though (I know this may have some overhead on the list server... If that's the case, I'm sorry I suggested this).
Sorry, I am a stubborn person. I still want to use HTML. If you can show me that this list REQUIRES its list members use plain text, I will follow the rule.