How does the DiggBar dynamically resize its iframe's height based on content not on their domain?

Question

Someone has already asked, How does the DiggBar work? in a previous question.

While someone provided a decent answer it didn\'t address one thing:

Answer 1

100% in an iframe is a 100% percent of the declared parent space. An example is:

/* parent element */

html, body {
   width: 100%;
   height: 100%;
}

/* child element */
iframe {
  width: 100%; /* this is truly 100%, try it out */
  height: 100%; /* try it out */