It's certainly fun to look at, but I'd have a hard time following any of the mathematics. What I'd like to see is an online calculus book which takes advantage

It's certainly fun to look at, but I'd have a hard time following any of the mathematics. What I'd like to see is an online calculus book which takes advantage of interactive animations and graphing to demonstrate how things like integrals and derivatives work. For example, for the limit-based definition of derivatives, you could concoct an animation showing how the math works as deltaX goes to 0, and so on. There's also clever ways of demonstrating the product rule through geometry that make the math make so much more sense. There's a lot of potential in interactive, graphical textbooks that has yet to be explored, I think.