Back in the late 90s, when Netscape's email client supported it, I used iframes in email content to display the latest/current state of a complex workflow. We

Back in the late 90s, when Netscape's email client supported it, I used iframes in email content to display the latest/current state of a complex workflow. We used emails to notify people of some decision they had to make, and needed an obvious way to avoid someone doing the same action twice without having to click around and leave the email client. The iframe would be populated with content from a webserver showing the current state. The first time they opened the email, they were able to take action immediately. If they later opened the same email, it would show the current state with the result of the action. If multiple people could take the same action and got the same email, the email would appear to always show the latest state and it avoided some workflow confusion issues.