How to build a room where using can interact in a completely static website

For a project I am supposed to create a static website (to host on GitHub pages), but my goal was to create an in-browser game where users can play against each other in real time. This isn't too difficult using server-side languages like Flask or Django, but I am really struggling if there is any way to do it with only JS, HTML, and CSS.

I am looking int Pusher, but I don't know if that works without server-side code.