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Reminds me of a community I used to be a part of, when I played the online game Utopia. Over at www.avidgamers.com they provide basic types of communities for alliances and stuff like that, in different games.
Anyhow, those sites do clean out certain markup in a pretty good way - the administrator can choose which tags are allowed - scripting is never, and from what I can tell, the engine that strips such is pretty good.
But they had allowed the <table> and the <div> tags. So me and a friend started impersonating other friends in the forums, mostly for fun, but also to enlighten this problem. We basically did this by creating posts that contained closing </td></tr></table> that were the same as the forums, and then built up a new post "after ours" within our own post, having those guys saying really funny stuff. It took almost two days before the HTML illiterates (no wrong with that, mind you) figured out what the *** happened.
With <div>, we created signature boxes that hung under the menu - something like "This thread contains a post by XXX!" with lots of colors and stuff.
Most people laughed their heads off while we were rummaging around like that, but some did take offense - none that we impersonated though. And it was really funny to see people saying " I did not say that!" when everybody could "see" they did.
Then, of course, we told them which tags to turn off, and no more problems.
You have moved into a dark place.
It is pitch black. You are likely to be eaten by a grue.