tye,
On 2006-11-21 at approximately 09:45 EST, all font turned bold. All attempts at correcting the problem failed. Unlike before, switching from perlmonks.org to perlmonks.com and closing all the affected tabs did not work. This is FireFox 1.5.0.8. I now have to completely close my browser.
Frankly, that sounds like a browser bug. FYI, I don't see any avenues for me to investigate. If this happens again, you might want to have your browser show you its impression of each relavent CSS page for comparison to when the problem doesn't happen. But my wild guess is that there won't be any difference. You could try using the DOM inspector / javascript to try to figure out why your browser has decided that a particular chunk of text should be bold (or perhaps install a CSS debugger add-on). Or construct static HTML pages that reference the same PM CSS and see how those behave. (I don't have access to do any of those things since I've never seen this problem.)