There are no standards for HTML email, and this bugs me every couple of months when I have to design a new HTML email template because there's so much testing involved across different browsers, web mail systems and installed clients.
It would be so much better if everyone worked to an agreed set of standards, which is what the Email Standards Project is trying to do. http://www.email-standards.org
One day I'm hoping everyone will support CSS embedded in document head, but until then it's fiddly in-line styles on just about every markup tag. Still there are many that don't think we should be even trying to style HTML emails, (Jeffrey Zeldman who I hold in high regard being one) and say it's just a message format.