After much research, at least at this point there is no way to include html tags in the mailto body. If a particular email client plays nice then you are in luck, otherwise there is no standard, or cross email client way to include html tags in the mailto body.
During my research (just googling around…), I found this nice tool that can help construct the mailto link. You simply need to fill in the to, cc, subject, and body, then the mailto link will be generated for you. Check it out at http://jscode.com/generators/mailto_generator.shtml