Very nice artwork. I found the default controls quite frustrating and changing them in the browser was difficult, the game didn't recognise my controller (Xbox One controller) :(. The music is fitting. I didn't enjoy how long the bosses ran for. I loved the stop motion effect with the low frame rate animations. The bosses needed to do more telegraphing and the shield should block immediately rather than when it's fully up as it's quite frustrating thinking you've blocked a projectile only to have it hit you. The sound effects work well with the visuals.
I think for the second boss a way to improve might be to have the small spinning tops shooting a specific colour each and the big boss just doing dash attacks. I think an animation to show when the colour will change would be a good thing. I also feel that when it dashes into you it should knock you away, I kept taking multiple hits of damage from the dash attack.
I think for the third boss having some kind of ranged attack would make it feel more fair and again, telegraphing to know when to stop attacking in the rare moments that you CAN attack would be nice. I think the hitboxes on the pages need to be a little more forgiving.
I-frames would be good for when you've already taken damage and maybe allowing the attack animation to be interruptible would make the game feel less frustrating. If you want to keep the uninterruptible animation I think more telegraphing is a must.
Finally I think, unless you were planning to have a 4th boss that requires the use of all of the abilities, having to switch manually to the fast dash on the third boss doesn't make too much sense as the default reflect ability seems to have no use for that boss. (I was a bit too frustrated to finish the third boss so maybe there is a 4th boss?)