I strongly vote: NO on HTML.

For many reasons: People could easily include / embed ads. Browser
incompatibility / accessibility issues for those with lesser eyesight
/ large fonts by default (usurped by the html post).



As for top-posting: YES.

I think it's actually useful. You should post a response related to
the relevant thread of discussion. I'm sick of reading thru 10posts
about a flame or random thing, just to get back to the original thread
of discussion.