> Is there any real life examples of achieved speedup?

Yes, most pages have significant speedups during the initial restyle especially. Wikipedia pages tend to get 3x or so improved style recalc time on typical systems with Intel quad core CPUs, for example.

Of course, styling is only one part of the whole, so your overall speedups are limited by the rest of the rendering pipeline. That's Amdahl's Law for you. But that's no reason to not parallelize at all; it just means that we have lots more work to do once this is done :)