Something I find interesting, is dt is directly proportionate to fps.
High fps = low dt = more frequent ticks = lower fps = higher dt = less ticks = go to beginning.
What does the above do? Cause a speed up/ slow down.
For me, I can only get this effect above 60hz. And the pattern looks pretty consistent between dt/fps graph.
Unless a browser maintains a solid vsync/refresh and a machine can provide the fps to match you get jitter.
Attached picture shows the DT spikes correlate with FPS drops. Which causes DT to drop/stabilize. And Repeat.
Still though. This effect is not the same as the OP. This has more to do with my creating an object every tick. But it could contribute to the effect in some projects. It also shows some issues with fps/browser vysnc.
IE at 121hz lowers to 60fps. As a result it is stable, does not have that effect nearly as much as chrome.
Chrome however performed amazing at 60hz where IE then went above the 60hz refresh.
And one more picture.
Please note: in this image the blue fps graph is still technically higher than the red dt graph. The red graph is essentially enlarged/zoomed in.
The dt spikes still correlate to the fps drops.
It also really shows how unstable dt is. Where we would ideally want dt to be constant.