sizcoz Tested your export. These are my results:
Surface pro 4:
Firefox: See very noticeable janks.
Chrome: 69, janky.
Canary: janky but a bit less.
Now here comes the twist though!
I connected the power cable because my surface was getting a bit low on battery. Hmmmmm....janks suddenly decreased quite a lot. Chrome 69 and firefox still had some, but not as frequent, and not as heavy. I thought, that's odd so i tried something else. I clicked the battery icon in the task bar, and changed setting to best performance.
Result after setting:
Firefox: no janks
Chrome: no janks
Canary: no janks
Are you testing on a laptop? or using balanced power scheme?
Results on Sony Xperia XZ1
Chrome: Some very rare janks far between.
Edge: Some occasional jank.
So on windows, power mode setting seems to make a huge difference. In other desktop games(non html5) this usually only lowers framerate when on battery, but does not cause janks.
I only have laptops, tablets and mobiles to test on. No regular gaming computer, workstation or anything like that.
I'll test on macbook pro when I'm back at work tomorrow.
I'm on laptop, power cables always plugged in, but guess what, I went to additional power settings, and set up a custom power plan, and set it to High performance, and got very minimal jank.
Next dilema, persauding your users to ditch their powersaving modes.