Not bad at all, for something that has been possible in Java, Flash, ActiveX, PNaCL before.
Thanks to service workers, the miner won't go away when you close the browser, as by default settings (which normal users don't even know they exist) service workers run on their own processes.
You say possible, but where were they? Actionscript never ran at native speeds or even close to them. These things were never seen in Java applets either.
Webasm + webgl is a potent combination. C and C++ libraries can be used directly instead of trying to squeeze fast matrix and vector math out of a Java JIT that needs special wizardry just to avoid heap allocating everything.
And let's not forget that you implied that someone had to be careful of what sites they visited when it is actually a case of 1 in 10,000 sites carrying mild consequences and not security exploits.
I get that you love Java and don't know modern C++, but denying reality doesn't change reality.
Thanks to service workers, the miner won't go away when you close the browser, as by default settings (which normal users don't even know they exist) service workers run on their own processes.