That’s basically just an experiment, but it’s pretty cool, I think. It uses the ActiveX control version of Mozilla, embedded in a webpage, just like any other object could be. This creates a little window on the page which renders the URL using Mozilla’s HTML engine. You can use it to test pages for compatibility, and to see just how broken some stuff in IE is. The default page is Eric Meyer’s css/edge, which comes up with cool things to do with standard CSS. Unfortunately, IE is know for very poor CSS support, and the IE button on the page will let you get a side by side comparison of the two HTML engines.
You do have to download and install the Mozilla ActiveX control for it to work. http://www.iol.ie/~locka/mozilla/control.htm#download has the download links. Just download the file and run it, and it will install the control for the page to use. It’s about 4.5MB, but you basically don’t see anything once it’s installed. You don’t have to bother with using another browser or anything, it’s just a behind-the-scenes object that can be called. You may have to adjust the security options for some ActiveX stuff to view it, especially after WinXP SP2. If there’s any ActiveX stuff disabled, change it to “Prompt” instead. That way nothing else will be allowed by default, but you’ll be asked about it instead of IE just silently blocking it.