It's confusing that "Checklist" (where you check items off) and "Discover" (where you find explanation about each item) and "To Do" (where you can add your own items) are three separate pieces. It would be much more elegant to have them combined into one simple list / app.
An aside: If you click on the airplane on Mac, the window goes full-screen. (Not what I expected, either, but surprising.) I didn't know you could implement that on a site, but apparently there's newer HTML5 API for requestFullscreen()