Connect an xBox or PS3 gamepad and you will see a visual representation right here of every joystick movement and button press.
Make sure that you're using the right version of Firefox and have installed any necessary drivers for your gamepad and OS combination.
There are a few bugs and aspects of this demo that are known not to work:
You can download the source code on GitHub. Feel free to modify it and please send a pull request if you make any substantial changes that should be implemented here.