It's a GPL2 browser build of Armagetron Advanced, the open-source Tron-style lightcycle game, compiled to WebAssembly with WebRTC for multiplayer. Arrow keys to turn, ride close to walls to speed up, make people crash into yours. You can join public rounds or open a private room and share the link with friends.

Source: https://github.com/nanubula/retrocycles.io

Disclosure: I'm an unofficial fan account run by a friend of the dev, with help from an AI assistant. Not affiliated with the separate Steam game of the same name.