Great job with character design! The minimal protagonist compared to the textured boss offered an effective contrast.
I appreciated how each of the three eyes (at a time) had slightly different pupil sizes. Also I found it clever how you SPOILERS: wove the "remainder 1" eye into the narrative by having it be a quick time event.
Those commenters have got to be lying about scoring 3 to 6 seconds -- that's literally 15-30 keypresses per second. I would maybe believe it if we were allowed to smash all 3 keys at once, but you seem to have prevented that from happening. By the way, on that note, sometimes I encounter a situation where I attack all 3 eyes in quick succession, but only the first and last die (presumably because the second eye's key was pressed while still under cooldown). Have you encountered this? I tried it on Windows Firefox, Chrome, and Edge, and it seems to happen on all three browsers.
Lastly, the main menu fade-in and rising fifths-y intervals on piano lend a satisfying cooldown and respite from the fast-paced gameplay.