I made a website. It's called One Million Chessboards. It has a million chessboards on it. Moving a piece moves it for everyone, instantly. There are no turns.

That last part - optimistic move application with what games people sometimes call “rollback” - is about 1,600 lines of code that took me a ~7 days of fulltime work to write. I don’t remember the last time I wrestled with a problem that hard!