If you go to this page by Greg Taff, you'll be able to edit the page code and CSS in real time.

"What's the point," you're wondering? This page is a proof-of-concept, meaning it shows that you can do something. In this case, it means that other people can use this code to create on-the-fly web editors. The uses of this code aren't really apparent right now, but some enterprising soul will figure out how to make money off of it.

There's also a Greasemonkey version for you Firefox users. Hmm, I guess somebody's already figured out how to use it.