Ah, browsers. Vsync timing is basically impossible to guarantee with them. It's dependent on which browser, browser version, which OS, and I suspect even which graphics card and driver (because OpenGL is affected by those).
Thank you for that, I wasn't aware that site existed, already confirmed some of my worst suspicions. What we are working on is a music application that horizontally scrolls a score drawn on a canvas. Smooth scrolling is quite essential to be able to keep to the beat (see: https://news.ycombinator.com/item?id=24615125)
I use this to asses:
https://www.vsynctester.com/
but this doesn't give me a feeling that all browsers in common use will do "buttery smooth" any time soon:
https://www.vsynctester.com/firefoxisbroken.html