How to unstyle incorrect (bold and italic) font display in Safari and Firefox?
When I view this page in Firefox (12.5 beta) I am seeing this text as you probably intended it - no bold and/or italic. Since you reported that the problem is occurring in both Safari and Firefox, it likely is not user preferences in both browsers, which can be set to use user-defined fonts instead of those dictated by CSS or HTML instructions. Nevertheless, I'd check each browser's display preferences first to see if this isn't the case.  

Otherwise, this may be a problem with your fonts cache, so clearing it may solve your problem. You can easily do this by opening up Terminal.app and entering the following command (this clears the font cache for the current user account; if you have other accounts, they won't be affected):

atsutil databases -removeUser