Steps to reproduce:
1. Open 2 blank tabs in IE.
2. In one of them navigate to this demo page: http://jsfiddle.net/MTJ27/15/ ; it runs 4 web workers.
3. Check CPU utilization (on a 8-cores machine it should show 50%).
4. Close the tab with the demo page, leaving the other tab open.
5. Check CPU utilization again -> it doesn't drop! IE still runs web workers.
Nasty, isn't?
Regards,
Robert