Two browser windows (acting as socket clients) communicate their:

• Screen dimensions - (screen.width, screen.height)
• Window dimensions - (window.innerWidth, window.innerHeight)
• Window X/Y position - (window.screenX, window.screenY) ...or whichever calculation works best for you.

The original post says Bjorn Staal used localStorage, but I found sockets more fun, because if tweaked a bit, this can be shared with friends :)
Here's a demo of how it works, and the codebase:
https://github.com/Momciloo/fun-with-sockets/