CAPTCHA without cookies: a proof-of-work approach

We've all been there. You're trying to sign in, you click "I'm not a robot", and instead of a simple checkbox you get a 3×3 grid of blurry photos. Click all squares with traffic lights. You miss one — was that a real traffic light, or just a pole with a light on top of it? Wrong. New grid. Crosswalks this time. By the third round you've forgotten what you were trying to do in the first place. That's the visible part. Behind it, there's something less visible: a small army of cookies and trackers