HTML in email

Steve and I were talking this afternoon about HTML in email. He wanted to know what headers I looked for in the HTML portion of an email. A good question, as I’ve seen everything from a full doctype declaration through to just <body> tags.. All of them seem to render OK in various mail clients so I don’t spend too much time worrying about the specific HTML header elements. I do look for invalid tags and comments, but I check those whether they are in the header or the body.
Those of you that design HTML emails, what are your experiences with headers? Are there specific HTML headers that you always include? Do you skip the header portion of the HTML document and just use body tags? How do you test? What do you think is important?