I code a lot of Eletters. It’s one of the biggest challenges that I face as a designer.Knowing which ones take padding or margins or can I use <div> or do I have to use the <br> or the <p> tag.
Campaign Monitor is a huge help wiht these. They have a great resource tab that lists all the mail email clients and which ones accepts the code. Based on your readership, you can spend most of your energy on those clients. And this CSS list is the best.
Another VERY COOL site is Premailer!
This site will take your <head> styles and make them inline for you once you are done building the template!
You still have to be wary of what styles function across all email clients, but what a nice way to not have to type in each line of code for the initial design! Everyone say yay for a huge time saver!