“An on-page advert was mining bitcoin in my browser”

That's a reasonable theory but I don't think it holds. Given that (1) $ARBITRARY_ALT_COIN aren't typically well-known, (2) setting up mining may require different hardware/software to enable computing this PoW, that pool, etc, (3) coins and pools have interesting reward & difficulty adjustment algorithms, some better designed to deter coin-hopping. On some coarse-enough scale the general claim of equalization is reasonable -- the differences will be minimized over time. But I have seen times when you could make 5-10% more by mining $ARBITRARY_ALT_COIN (and perhaps taking the altcoin to exchange for bitcoin).

> There's no need for that: https://github.com/jwhitehorn/jsMiner

shrug, seems like we're on a tangent. This thread was speculating about whether the claim of an ad-served-miner exists. The fact that a small JS miner exists for one PoW doesn't seem to rule out the original claim, nor does it rule out the possibility that a wasm/asm.js cgminer might exist.

Thats.... what people do.

Market inefficiencies come up predictably because coins have different block reward and emission schedules. They have different algorithms which give advantages to different processors. The market efficiency time sometimes requires a completely new fabrication line created in china, this gives significant times till mining arbitrage goes away.

Similarly, multipool miners have software which automatically switches to different blockchain networks when the yield is favorable.

It is a mistake to think there is no opportunity just because someone told you there are always smarter more observant people profiting off the opportunity.

I guess I'll help:

The costs in this case would be the time and cost of programming and bundling the ad, as well as the ad spend of getting the ad shown. The earnings would be in cryptocurrency. Any non-zero amount of cryptocurrency earned would mean you have a return .... on investment, and any cryptocurrency amount now or in the future that is valued at more than your costs means you have an ROI greater than zero.

Mining pools count shares of work toward the pool itself finding a block, based on processing power contributed to a pool. So it doesn't matter if users uncovered and uninstalled your mining-ware after 5 minutes in the most technical savvy outlier case, the shares of their processing power was already submitted to the pool and that account got paid.

So wouldn't it be hard or almost impossible to say they have an ROI of zero? They are probably making a good amount of money.