Since we are in the age of CSS3, you can do this by using viewport units. These units allow you to specify sizes in terms of percentages of the viewport width and viewport height. This is the user's viewport, also known as screen. However, in all major browsers I've tried it, if you put an iframe inside a div, which is inside another div and positioned relative, the viewport units are relative to this div. And since 100 viewport height units mean 100% height, you can do like this:
<div id="parent"> <div id="wrapper" style="position:relative"> <iframe style="position:absolute;top:0px;width:100%;height:100vh;" src="http://anydomain.com"></iframe> </div> </div>
And most importantly, it works on all browsers, even mobile ones (tested on android and iphone)!