Check out fractal or the inline styler, which converts your embedded CSS to be inline and also gives you a report on your code’s compatibility with major email clients.
Keep in mind that these tools do not check for unclosed tags or other standard errors, so it’s a good idea to also run your email through the standard W3C validator.
Also, Campaign Monitor has written up a nice post about the use of DOCYTPE in email.
Once you’ve run your HTML through the validator, you’ll receive a list of warning and errors.
An element is considered deprecated when it is slated for removal from the specification when new or better ways of doing the same thing are introduced.