We prototyped a JS version of that during a hack day, with the intent being that people could elect to donate some mining time to charity while using a client's website.

It worked, technically, but the return was basically zero.

That's too bad. It was always something I wanted to prototype but I had a feeling it just wouldn't be worth it. Do you have any statistics like average BTC produced per 8 hours of browsing or something?

I don't remember off-hand; it wasn't my team. I can dig them up if you'd like though, fire me an email at [email protected]. IIRC it worked out something like a few cents for our entire (sizable) user base.

CPU/GPU mining of Bitcoin is a thing of the distant past.