1) It's amazing how a bunch of well-thought-out rules can produce complex, powerful and consistent behavior. Here, we have basic structural editing, enforcing structural correctness, and Python-like "structure by indentation" editing, all with no hotkeys other than one to switch between "indent mode" (you control the indentation, parinfer controls parens) and "paren mode" (you control the parens, parinfer controls indentation).
2) The page is a stellar example of documenting interactive functionality. It starts with an animated demonstration up front, continues with explaining the value proposition, then gently introduces background information necessary to fully understand the value proposition, explains the high-level abstractions behind the product, and ends with detailed usage examples. Everything that can involves an animated example - but it's not just an animation; all those textboxes are editable, so you can interrupt the animation to test the functionality live.
2a) On that last note, it was a brilliant decision by the author to build parinfer around formalized principles and a reference implementation "in straightforward, imperative JavaScript, optimized for speed and designed to be easy to port to other languages", with "a test suite which is also designed to be easy to port with all test cases represented in JSON files". This is why it took no extra work to provide interactive demos on the webpage, and it ensures the product - which is really more of a concept than an artifact - can easily spread.
I think it's worth learning from the author, even if one isn't interested in Lisp or modding their editor.