Nice game, cool art and good enemy variety and behaviours. The game starts completely silent though, you might want to get the music going from the main menu screen.
One thing that I've found a bit weird and had to double check was the "Start" button, clicked it some times and didn't seem to react. I mention this because i didn't see anyone else doing so. It looked like i had to click close to the letters of Start (or directly on them), while usually there's sort of a activation box around the text.
I did refresh the page to make sure and it happened again. In example, try clicking the empty spaces inside the "S" of "Start" and you'll see what i mean.
I hope this helps, cheers ;)