>>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" :).
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>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.

What I had in mind is to use something like

outer text <span class="italic">text text text</span> more outer text

instead of the I tag, and similarly for other tags. Nothing changes in the interpretation of older text, just these classes (defined somewhere in your .css) would apply to text formatted using these buttons in new messages.