I’ve been using Safari Tidy for a few days now, and recommend it highly. The author says:
The Safari Tidy plugin is a small plugin that lets you validate the webpages you browse for (x)html compliance. The actual validation is done by Tidy. This plugin was modeled after a similar plugin for Firefox, which can be found here.
Download Safari Tidy here
I’m finding it especially handy for WordPress, both for my own markos, and for cases where WordPress’s automatic markup conflicts with my own. I assume that the Firefox version would be just as useful.