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It should be noted that their intent was to be purely opt-in. Stealing a user's CPU/GPU cycles is extremely bad form. (It's bad enough when ads/tracking code does it by accident.)

CPU bitcoin mining doesn't really do much. The idea is nice, but it would take way too long to be competitive as a form of revenue generation if all it had access to was the CPU.

i'm willing to wager it's possible to write a WebGL shader to mine bitcoins.

It's been posted before and quite frankly, it can very easily be abused as a malicious feature. Especially if the user is unaware of their being complicit in mining.