We can find a large number of iframe-autoresize scripts on the Internet. But I found no script that meets both of the following requirements:
- the 'seamless iframe' must be dynamically insertable; that is, it must not be 'already there' when the page opens;
- resizing must be animated.
So I wrote a tutorial on how to produce dynamic/animated auto-resizing iframes.
The link to the tutorial is HERE.