fwiw WebKit never turned on the threaded html parser (and deleted it after the fork [1]) and Blink removed it a bit ago too [2]. In Chrome we measured it across a number of sites and found that the time saved from background tokenization wasn't benefiting real world content enough to justify the cost and that in some situations it actually made things worse.
That's not to say Servo shouldn't try it of course! Part of having a healthy multi-browser ecosystem is each browser trying lots of ideas and coming up with new solutions to problems that were encountered in other implementations.
WebKit/Blink put fewer parts of the HTML parser off the main thread, so negative results should not be taken to apply to more comprehensive approaches.
That's not to say Servo shouldn't try it of course! Part of having a healthy multi-browser ecosystem is each browser trying lots of ideas and coming up with new solutions to problems that were encountered in other implementations.
[1] http://trac.webkit.org/changeset/162260/webkit (the number cited here is a joke :P)
[2] https://groups.google.com/a/chromium.org/forum/#!topic/loadi... (see also the design doc linked from the email)