This is a horrible, horrible hack and it's terrifying and I am aghast at how amazing this is. Such a wonderful abuse of OpenType contextual alternatives. Well done and thank you for sharing!!
Putting animated SVG in a font [1] is a horrible hack; this one is benign.
A somehow larger stretch would be a font with a built-in spell checker, with a wavy underline under every letter that only disappears under correctly spelled words.
Agreed, that is even more horrible. And probably just for performance reason alone should never ever get close to a real world product. Still, quite neat.