you can do it just like designing your own webpage, but remove the <head></head> <body></body> tags and all images must be absolute image urls taken from your hosting space so it will appear in the email. I doubt the iframe will appear on an html email.
if the email only receives or is set to receive ascii, your html message will appear only as code not appear as html so this type of email message are seldom used.
remember the goal here in broadcasting email is to get your message across.
hth