The feel of the game was great. I enjoyed the music. The game was fun. As constructive feedback, I noticed that when I moved left it would auto-face me to the right when I let off the control. It would feel better if you allowed me to stay facing the direction I was last facing, if you are going to allow me the freedom to move in both directions. I also did not notice the other world mechanic until second run through. Mostly my fault I barely read while playing an action game. However, I could just jump over the shadow units and kill the physical ones that how I won my first pass through. You may want to not allow someone to pass without first engaging with the shift mechanic. I did not fully understand I shifted when I pressed the button on accident, I initially thought I had bugged my browser because the whole tinting effect is like what happens when you drag select in the browser.
That being said those are just friendly critiques. I did enjoy the game and played multiple passes at it because my jump skills are only so/so but I love me a good platformer. Keep it up nice work!