Update: Mark has published a comprehensive follow up to this article which takes a more accessible approach and includes a sample template to help you get started.
Most people who’ve attempted to recreate a sophisticated design in HTML email have run into a wall when using CSS, either in the form of inexplicable mangling by email clients or a pronouncement by an email administrator stating that CSS is “against the rules”. If you’re not content to roll over and use font tags in your HTML emails, read on.
Despite prevailing wisdom to the contrary, you can safely deploy HTML emails styled with good old-fashioned CSS. Yes, we really just said that. Not all attributes will be invited to the party, but many of them work flawlessly with this method.