Generally I tell clients to avoid HTML e-mails. Many servers (like mine) IGNORE them if they don't have a plaintext fallback (via multipart) for the simple fact no legitimate e-mail I would ever care about has ANY reason to have HTML in it... so automatically they're just spam.
If you HAVE to do it, again send multipart with a plaintext fallback, and stick to HTML 3.2 as if CSS doesn't even exist. Very few of the new HTML 4 tags are supported by all mail clients, CSS is often flat out ignored...
You basically have to dial the clock back to 1997, which is just ANOTHER good reason to not even bother with that nonsense. You want HTML, make a flipping page on your site and send a plaintext with a link.