<p>How can I scale the content of an iframe (in my example it is an HTML page, and is not a popup) in a page of my web site?</p>

<p>For example, I want to display the content that appears in the iframe at 80% of the original size.</p>

I do not think HTML has such functionality. The only thing I can imagine would do the trick is to do some server-side processing. Perhaps you could get an image snapshot of the webpage you want to serve, scale it on the server and serve it to the client. This would be a non-interactive page however. (maybe an imagemap could have the link, but still.)
Another idea would be to have a server-side component that would alter the HTML. SOrt of like the firefox 2.0 zoom feature. this of course is not perfect zooming, but is better than nothing.
Other than that, I am out of ideas.