Why are they so obsessed with 60 fps? 120 fps looks considerably better, and there are other effects like smear and judder that significantly decrease even with significantly higher frame rates, say 480 fps [1].
The WebRender folks are well aware that higher framerates are the future. Here's a tweet from Jack Moffitt today, a Servo engineer (and Servo's technical lead, I believe): https://twitter.com/metajack/status/917784559143522306
"People talk about 60fps like it's the end game, but VR needs 90fps, and Apple is at 120. Resolution also increasing. GPUs are the only way. Servo can't just speed up today's web for today's machines. We have to build scalable solutions that can solve tomorrow's problems."
As everyone said, 60fps is not the destination but merely a waypoint. It's a good goal, considering 99% of screens that are in use today refresh at 60 Hz or their regional equivalent. Higher refresh rates are next.
Not an expert, but I feel that that was more of an analogy/image to give what they were aiming for. The real objective is not 60fps, the real objective is to use the GPU to do tasks that it was designed for. Plain and simple. This however, gives the user a smoother experience, and 60 fps generally gives a noticeable difference.
We're not as other people have said in other comments. On normal content you can often see WebRender hit 200+ fps if you don't lock it to the frame rate. To see this for yourself, run Servo on something with -Z wr-stats which will show you a performance overlay.
I don't think, they are obsessed with 60 FPS, that's just what for most people is synonymous to a smooth experience and is often not met by browsers at this point in time.
In the video, he says 500 FPS, but assuming there's no more complicated formula behind this, I think it would actually be 2174 FPS. (0.46 ms GPU time per frame -> 1/0.00046s = 2173.913 FPS)
[1] http://blogs.valvesoftware.com/abrash/down-the-vr-rabbit-hol...