Cool game! I learned a lot.
One thing I would suggest is having an indication of score / number of correct answered. I got to the end, and all it said was "level complete." Maybe a five star system or a letter grade? A way to go back and retake the question you got wrong would also be nice.
Also, a quick fix: reduce the resolution of the game. I had to zoom out to around 40% size in browser to see the whole game, and I have a pretty big sized laptop. That should be a quick fix.
Awesome job! Keep up the good work and keep making games!