In that sample the foreground scrolls perfectly smoothly for me, but the background looks jittery. This indicates to me that it's not a platform issue at all. That sample is just doing something weird with the background.
This jitteryness is because the sample doesn't have antialiasing enabled (since it's pixel art) and the background scrolling is 0.1pixels per frame, which means every 10 frames it snaps 1 pixel. The scrolling is also updating on fixed amount per frame instead of looking at deltaTime, so if there are lags or small differences in frame time this might look choppy.
But I think it's more meant to demonstrate drawing parallax layers rather than subpixel scrolling.
Yes, the background is odd but the foreground is definitely not smooth. I see small little jitters occasionally. At one point I had to wait 15 seconds for it to jitter, though.
Yes the back and foreground is quite jittery for me on Firefox, and I'm almost certain its the browsers own requestAnimationFrame that's the problem.
Update: Although, having a closer look at the scene, I see its pixel art, so I bet the author is snapping floating point positions to a pixel point to prevent sub pixel blurring.
Another small update: I was sure requestAnimationFrame was locked to 60fps, but I noticed on Chrome the other day it was 144hz, the full speed of my monitor.
