Nice visuals! The gameplay mechanic was simple and kept me coming back after I died to try again :)
The audio didn't play for me for some reason and the powerups didn't work either (I'm using Chrome on a Macbook Air)
Would be nice if after you destroy the boss all his bullets also disappear, I took a hit from one of the boss bullets after I had killed him. The enemies are also a little bit hard to see sometimes especially the smaller ones when the screen is full of bullets. Maybe they can be more high contrast, or have some sort of visual indicator to help them stand out (outline maybe).
Fun game overall, I enjoyed it!