If you're interested in testing on browsers with their heads intact, have a look at the Watir project, for which Opera has just released an implementation to complete the set: http://operawatir.com
Disclaimer: I work for Opera.
However, there ought to be a way to dump the DOM as HTML that can then be viewed in a browser. If the feature doesn't exist, it couldn't be all that difficult to write.
Side note: This is the same guy who gave us Vanity! Thanks!
2. Capybara (currently) uses envjs for headless full-stack testing.
3. JSDOM, used by Zombie.js, certainly has different characteristics that may make it better/worse under some conditions.
4. If you're thinking Ruby, I doubt that any gems currently integrate with Zombie.js. However, that is also (somewhat easily) changed.
Sadly, full-stack testing in Ruby still kind of sucks. We spent a whole session at Ruby DCamp 2010 discussing the state of affairs. As much as we'd all like a robust headless solution, at the moment, the best browser simulator is still a browser.
YMMV. I've had mixed results with env.js. While not as sexy, I've been using Capybara with the Selenium driver with more success lately.