It looks perfectly smooth to me. I've seen a lot of these kinds of tests over the years, generally aimed at v-sync quality - they tend to look fine for me these days. For example on my system the space blaster vsync test can hit every frame within 0.1ms over a period of a minute. There does seem to be variation across systems as to how liable it is to jank.
FWIW GC is entirely on the browser's end, but in modern browsers it's all designed to run in tiny slices of just a few milliseconds to avoid jank. It shouldn't be holding up for a noticable amount of time.
It could actually be a browser addon spewing out rubbish in to the GC - have you tried disabling them?