This will never become a thing. If my phone heats up and dies after twenty minutes because the news site I'm reading is crunching numbers on my phone's GPU, I a

This will never become a thing. If my phone heats up and dies after twenty minutes because the news site I'm reading is crunching numbers on my phone's GPU, I am (and most other folks on HN are) going to be VERY salty. Compute time on user machines isn't free, and users will notice. On top of that, display advertising pays when the user sees the ad. Coin mining only pays when the user has completed enough useful work to send results back to the server. The odds of that happening for a huge number of websites are very low.