The Pirate Bay just tested that yesterday, apparently. They bundled a Monero javascript miner with some of their pages. The funny thing is that while the legality of that is questionable enough for the more reputable websites not to go down that road, TPB is already shady enough to pull it off - and their users don't really have anyone to complain to.

It's worth noting that this isn't exactly a new idea - some MIT students had a project a couple of years ago that was basically that, except they tried to turn it into a startup. The viability of it was questionable back then as well.