This is great stuff. They saw Bret Victor's talk, "Inventing on Principle" and took it seriously. The site looks great. You can modify variables on the fly and play a tutorial that will update the code as the tutorial plays, pause that tutorial and inspect the code, continue, etc.... Look how they have even incorporated documentation below the editor which links to actual running code. When people have questions about a specific bit of code, they can ask right on that page. Can't wait to try out more. Kudos to Khan Academy, again.