Brian,
I think the problem with the HTML display is a function of the webbrowser control used by Outlook and the fact that, for security reasons, Outlook intentionally disables certain script and I believe CSS functionality.
You might have better luck with one of the following techniques:
1. Ship your HTML message as an attachment vs. including it in the body of your email.
2. Generate a PDF vs. HTML message that gets shipped as an attachment
3. Email a link to your HTML message that requires a user to visit your website and retrieve your message as a standard web page.
Good luck!
Malcolm