Host a game on a not web instance, then press esc for the pause menu.
In the pause menu, under the header that says "paused" or something like that it should tell you
your local ip address. put that into another not web instance, and you can play together.
If you only have one computer with the game downloaded, you can run a web instance locally by downloading
the WebGL version and hosting it on a webserver on your computer.
The game runs on port 26492 if you need to port forward.