This article is useful when comparing tags that are supported by various clients.
And here's a code validator from mailchimp:
www.mailchimp.com/add-ons/inboxinspector/ (too bad its not free)
Before gmail became popular, alot of developers where putting the style tag inside the body tag, however since gmail now strips this out, unfortunately, the only sure way to handle CSS in your email is to do them in-line.
