Worth noting that lichess and all of the code behind it (some of which is algorithmically very interesting) is developed in the open[0], and Thibault Duplessis even regularly livecodes on Twitch[1].
I discovered lichess when I was looking for JS framework for a project and found mithril.js, where they referenced it as a successful user of the project.
hell yes, I've been playing those little chess puzzles for ages. Extremely addictive!. Neat that you can drop into the game and "continue" from a point when you're just _sure_ the computer has it wrong, only to find out that of course it was correct all along.