>How about introducing the formatting buttons? There's lots of free code doing that in various forum platforms. Then the actual markup may be just a span tag with a class of your own. There's room for that in the bottom right line of the message header, below message ID. Just the minimal set of tags - bold, italic, strikethrough - what people already use. So at least we wouldn't be seeing people trying to "use" quotation "marks" for "emphasis" :).

Yes, but not only do we need to validate on input but also on the output. I need to be sure that whatever is on the database, good or bad, doesn't break the page.