....you can probably sense the but coming.....but - this means any page you navigate to, any link you click, any random website with funny pictures of cats on, has unfettered access to your CPU. While you browse worldsfunniestcats.com, it could be using your electricity to generate BitCoins for itself, or crack passwords.
To prove a point, below is a page that spawns a few web workers and sets them, continuously, generating prime numbers. Open up the page in a modern browser and check out your task manager (closing the page kill the web workers):
Update: It doesn't max-out the CPU in Opera...why? Because apparently Opera is so fast that it can handle the workload without breaking a sweat!
Update 2: Apparently IE10 can hack it too, looks like firefox and chrome are trailing on this..... there's a sentence I never thought I'd write.....
Update 3: On further investigation, it looks like Opera's performance is a combination of running the script at great speed AND some kind of limits on web workers, either in terms of number or CPU usage. I can't comment on IE10's performance as I don't have a windows 7 laptop..... if anyone is listening at microsoft..... *hint* *hint*