Thank you for playing! :)
Yeah, I forgot to take into consideration that playing in non-fulscreen in some browsers may produce that undesired behavior (which I could had prevented using event.preventDefault() and event.stopPropagation() in JavaScript part, in keydown event handler function). However, it should work fine either playing with WASD (and J for jumping) or playing in fullscreen.
Nice to see the duck joke I inserted (which arose partially out of necessity because I was running out of ideas how to represent that upgrade with 8x7 1-bit pixels). :D