Most miners already use GPUs (or ASICs depending on the coin). WebGL would let websites get a relatively larger piece of the pie from dedicated miners.
I don't know what they would have to be mining to make cpu's or integrated GPU's make sense, certainly not bitcoin where even if you pay nothing for electricity to are unlikely to be able to do any useful work in the time someone visits your site.
PirateBay probably won't make any money. I'm considering adding this to an online game, which would probably generate a modest income. Video and social media sites could definitely benefit as well.
I think this would be fine as long as you have the users' consent. I suppose you could even give out rewards to players based on how much they contribute. I see this model in a lot of Android games nowadays (although with ads), you are asked to look at an ad/watch a 30 second video, and in return you get a small amount of in-game currency otherwise only obtainable through spending money.
In the discussion yesterday I estimated 2-5 cents per 24 hours of mining on a users machine (based on quoted hashrates and current prices). That's really not much, even if people actively let it run to help and there is a medium efficiency gain to have somewhere.
It's a card game simulator I have yet to publish. I don't really have a lot of money for server uptime, so it's either ads or this. And it won't be on by default. I'm simply offering users a way to support the site if they like using it, without having to view ads or pay directly (it may or may not affect their electricity bill I guess, but to most people an online game using some extra CPU isn't going to have much impact on them at all).
What about what he said implies he's doing it in a scummy way? Something like this, opt in and transparent, would be much preferred over microtransactions or ads, IMO.