One Simulator
I'm working on One Simulator (https://onesimulator.slidingcodes.com), a One Piece TCG simulator that runs entirely in the browser: online matchmaking, spectating, ranked, deck builder, replays (and alt cards you can flex on your opponent). Playing this game online used to mean installing something first: a client on your PC, an app sideloaded onto your iPhone, or a tabletop sandbox where you apply the rules yourself. And your opponent had to do the same before you could play. Now you open a link and you're in a match, on any device. The hard part turned out to be the rulings, not the netcode. I play this game in real life and I still had to look half of these interactions up, because what a card says and what a card does are two different documents. New sets land almost every month, and every one of them is a pile of new special cases to implement and test by hand. Which is why I now spend a suspicious amount of time wondering whether I should just train an agent to do it instead of me.