After a week, I fixed some things and improved others.

• Images are pixel-prefect.
• Added a workaround for gamepad.
• Show keyboard keys instead of pad buttons depending used device.
• Added an epic music for the boss-fight.
• Improved enemy spawn.

Gamepad is not supported on Chrome but Ok with Firefox. Anyway, there is a bug with pad button mapping in Unity for WebGL and the new Input System. Then I developed a workaround and it works fine now.