Cool game mechanic, and I really like the sound design overall and feedback/integration for the Z mechanic. Even with the simple graphics it makes the game feel really nice.
One thing I had some trouble with was simply getting past the start menu screen. The game loaded and showed the menu, but it wasn't apparent to me how to proceed. I tried clicking and pressing enter etc with no effect, so I gave up the first time and tried it in another browser thinking it was broken. Only there I accidentally found out up/down changed the selection and understood I probably had to press some other key to start, eventually finding the right one. So maybe you could give the player a hint about that?