I took the Pointerlock example from mrdoob (the creator of the famous three.js JavaScript library) and created a small prototype to showcase multiplayer and multiple viewport capabilities.
I included the node_modules and the js on purpose so that it should be possible to start it without downloading packages.
You need nodejs installed.
If you are lucky, you can download the zip or clone the repo, enter the folder and execute node server. The you should be able to see a page on http://localhost:8000.
If that doesn't work you will want to download the modules with npm install.