You are in the homepage room. Move, and everyone there sees it.

Drag your square with a finger, the pointer or the arrow keys. Anyone on the homepage right now sees it move, and you see them.

This is one more client in the same room as the homepage, which runs two more of them side by side. Each client can write only its own square; the server decides which square is whose. Your game gets the same room, with one script tag and one key.