From: Using CSS and HTML in Email Newsletters — Groundwire.
CSS and HTML support is far lower for email than it is for the web. This article explains the pitfalls to avoid and the methods to employ in order to get email newsletters to display consistently across many email clients.
Email newsletters are more popular than ever, and everyone loves to design a rich website-like newsletter for their subscribers. Unfortunately, email isn’t the web. Your beautiful design work might look great in one email client, but chances are when you look at the same work a different email client the results will not be the same.