Interesting, but the problem with writing HTML e-mail is rarely real-browser testing, but rather, testing the convoluted filtering/rendering of the various webmail clients (Hotmail, Yahoo, GMail).
It's like make HTML/CSS for a webpage. First level, you create you HTML/CSS code and test it with Firefox usually. Then when you are happy with, you check safari/chrome and debug IE :).
For instance, MailChimp takes CSS styles and modifies your HTML by placing your CSS into inline style tags. This is great, because you can have cleaner markup but still support clients that don't apply CSS well.
(I'm not affiliated with them in any way, just found the app to be very useful while working on a recent project)