Pretty sure all WebKit-based browsers (including chrome) do this. I'd guess they block if needed, but I'm not sure.
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)
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.