I'm just asking if the developer of a website can use the computing power of the visitors in somethings like cryptography and bitcoin mining..and if that is efficient for them or not..? I know that distributed DDOS attacks may launch by similar.. where can i read about that topic or something related...?
So I think DDoS attacks - that rely on the number of connections or the amount of data send (basically the bandwith of the upstream internet connection - are more likely applications for hijacked browsers. But yeah, you can use the resources that the host PC (to the script or application) makes available - of course.
This would be probably better as a comment but, I don't have the reputation.