At first I thought it didn't work for me neither in Firefox 49.0 nor Chromium 51.0 on Fedora 23.
One has to first press one of the "GAME A" or "GAME B" buttons, and then press the "JUMP" button.
One has to first press one of the "GAME A" or "GAME B" buttons, and then press the "JUMP" button.
(Game A / Game B was how you started the game on the original)