If you thought browser support for web standards was tough, try getting HTML email to look the same in every mail client. You can’t. Not even close. The best you can hope for is to have your design degrade gracefully in less-able clients.
You can’t just save and hit F5 to reload and view your changes, either. You have to click over to your mail client, compose a new message to three or four different addresses that you’ve got set up with different mail clients, and then wait and see what each one shows you.
I’ve had a bible and a wizard by my side to help speed things along, but it’s slow going – mostly because Outlook uses Word’s HTML rendering engine. I discovered that Word treats “border: 0px” as 1px, so I’ve been using that to my advantage to help provide basic formatting to Outlook 2007 users.
Composing HTML email is one of the hardest things I’ve ever done. I would not recommend attempting it if you’re on any kind of deadline.