"When exporting to HTML5 note that web browsers limit which servers you can connect to. By default you are limited to files hosted by the same server as your app. However if the server sends an Access-Control-Allow-Origin header in its response that allows your app's server, then the browser will let you connect to it."
This must be why the multiplayer is not working here at itch.io, but it is working on the server that is hosting the PHP script that accompanies this game. (sorry about that, but you can play the NPCs here)