I think that it isn't possible...
I can only suggest to you something that can give you something to getting started on it.
Jquery, on a big HTML with scroll, how to capture only the visible part of the HTML?
or you can use this plug-in:
jQuery.fracs, see the demo: demo
Probably if you want to show a portion of HTML and get only this visible part you must think up something and probably you can't use iFrame.