The suggestion for PoW for email bonds was called Hashcash, by Adam Back, and involved partial hash collisions.

http://www.hashcash.org/

It served as the inspiration for Bitcoin's PoW mining, interestingly enough.

The original invention of PoW, as well as the idea of using it for email, was years earlier; see Naor and Dwork's "Pricing via Processing, Or, Combatting Junk Mail" in CRYPTO'92.

Cloud flare does do this. You get a screen saying checking your connection and it’s running hashes in your browser.

is it really running hashes, or is it probing your browser to see if your video card drivers look like phantomjs

I was actually quite curious about that and I found this.

>With a JS challenge, Cloudflare presents challenge page that requires no interaction from a visitor, but rather JavaScript processing by their browser.

>The visitor will have to wait until their browser finishes processing the JavaScript, which should be less than five seconds.

https://developers.cloudflare.com/fundamentals/get-started/c...

WebGL, an important part of browser fingerprinting, takes a long time. Im sure there are other APIs being 'abused' for this purpose that take a while. This doesnt quite prove the PoW.

that permits both possibilities

Just like ads! They wear your battery but without your consent

Hey, that's not a fair assessment.

It would also dump greenhouse gasses into the atmosphere.

There is an app built around this concept called Bitmessage, but I think it's abandonware now.