IE10's chakra engine parallelizes interpreted javascript execution (on page load) with JIT compilation and garbage collection. So you can have up to three cores at work executing javascript even without web workers. Meanwhile, the rendering subsystem offloads to the GPU, and the whole thing can exist multiple times for multiple tabs, potentially involving dozens of cores executing your web apps.

http://blogs.msdn.com/b/ie/archive/2012/06/13/advances-in-ja...