Wow, that's the best I've felt all day. Truly lost myself in the piece trying to follow the red line and tease the individual notes out of my perception.
I will definitely work on getting that line synced up. On my computer (Mac OS X / Chrome), it's perfectly synced, but it seems that that isn't the case for some people.
FYI, I'm on a 2011 Macbook Air running Safari. It's good enough that I enjoyed the experience. Honestly, I was never absolutely certain that the red line was off. Just suspicious. :)
Here's the example I was using: http://notezilla.io/p/bach/air-on-the-g-string
[edit: I agree with crazygringo that it can be distracting if you are worried about the red line being slightly off.]