Earlier quoted context omitted.
https://friendlycaptcha.com/ Requires the browser to complete a Proof of Work challange. Not for verifying humanness, but against spam / dos attacks.
Proof of work is horrific. Means someones old phone has a meltdown trying to load the page while a warehouse in china with the latest GPUs cranks out billions of proofs per second. You could even utilize botnets to get random laptops and fridges to do your PoW.
Is that what this is actually doing though? Is it PoW for a mining operation, or just causing some electrons to be moved around to prove your not a bot? I didn't read too far into what the PoW actually is since their demo shat the bed.