What does Slither.io use protocol? websocet? udp?

I would like to develop a real time multi-game through a web browser. Simple chat can be implemented simply via websocekt or ajax but not real-time games. So I'm wondering how Slither.io games are made. Play with a web browser without installing it.What technique should I use? Do I have to use udp because it is real time? But does http webserver not support udp?Will it be real time enough to be a web socket?