I've looked at validator.w3.org to see if it was obvious, but validator returns tons of errors, many of which aren't real or serious.
So, challenge: send me a patch which fixes the HTML so talkpost.bml and talkread.bml with lots of comments don't look stupid on the mac. I'm not sure where the problem is... in the site template layer, or the HTML the talk* pages make, etc.
As this is a pet peeve of mine and I'm sure other people use macs also, I'll offer a permanent account for the first working patch. :)