I think you have to send the html document as an attachment. It needs to be MIME-encoded and the body of email was originally defined to be plain text.
It's good practice to provide both plain-text and HTML if you want to format email. A lot of us don't trust HTML email not to carry security exploits and thus have our mail readers set to display the text in preference to HTML.
Sometimes the only way things ever got fixed is because people became uncomfortable.