This might not be what you were trying to say, but it brings me to an interesting thought. Since you say 15%+ should be checked, would it be safe to say that 10% CPU usage, if these sites were to implement mining, would be acceptable to you?
I wouldn't mind a 10% CPU usage (as long as they inform everyone).
I agree though, more than 15% is certainly suspect, and if it is a consistent % of differing CPUs then it would seem that is a red-flag towards mining for sure. That might be a good test criteria, if you are suspicious of a site mining keep track of the % it uses of your CPU and compare it to another CPU, whether it be through another user or another one of your own devices. Would this work to any extent?
If by asking "if this works", you meant in terms of identifying their illegal usage of the CPU's, then the gathered information would be pretty much the same (that's why I mentioned checking the source codes [for accurate result]).
I wouldn't as well if I was informed. Call it as payment for using your favorite site. Or if they can have some sort of option to use the site like "CPU mining" or "see ads" kind of thing so we have a choice.
About the checking, indeed that should be done instead of just relying on the sudden increase in CPU usage which could just be attributed to your browser processing the website or whatever is in the background. Checking the code is more consistent and will lead to a better assessment of the presence of the mining program.
That's kind of alarming. These mining scripts haven't been out for a year (or maybe it has but has been hidden so well), and we already have that much sites running them. That's about 0.00136% of all websites if that was true.
