> I really hope that Servo helps to identify parts of the HTML/CSS specs (if any) that unnecessarily prevent parallelism.
This is beginning to happen; for example Servo work on running <iframe sandboxed> documents in parallel [1] led to the discovery that the HTML spec allowed different-origin iframes to mutate into same-origin, through the "document.domain" setter [2], which made process isolation infeasible (since it would require some way to merge separate processes with separate heaps back into a single process). That discussion led to a change in the spec [3].
[1] https://groups.google.com/forum/#!msg/mozilla.dev.servo/LQ46...
[2] http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2013-Aug...