I could work around the resolution issue by zooming out in the Chrome zoom settings to 50%, then using pinch to zoom on my Mac to make it larger again.
Super cool idea. I think it is unique and creative. To be honest I was a bit frustrated by having to trial and error the timing and gave up at some point. But maybe with some physics tweaks and level design maybe the game could feel less hard :)
Small feedback on the UI: I often didn't know what button was active, so I restarted instead of continuing the game. If you are still trying to play and the game is over you may accidentally press the right key and it will move to the restart button, even if I beat the level.