Code like it is the early 90s.
I would avoid all CSS with the exception of styles to remove link underlines if you want.
Nested tables & font tags are what you should be using.
Too many people advocate inline styles for positioning, margins, padding background images etc.
All of those should not be used in html emails firstly because of the lack of consistency across email clients and secondly it can get your email marked as spam which defeats the whole of the campaign!
I strongly urge you to avoid using CSS except where absolutely necessary, even then you will have to accept that it is not going to render as expected across all email clients. The only thing you can consistently rely on to have good support are your table tags and font tags.