The high contrast of the pictures, modern smooth scrolling, reveals the inherent ghosting (or overdrive) of most computer screens. While decent monitors are usually not noticeable in more regular use-cases, tightly-packed lines are very noticeable.
That means that there is a partially-faded copy of the previous frame still visible for a fraction of a second, which your eye sees, and the resulting image has Moiré fringes because of the mismatching angles.
For example, my monitor shows the purple drawings pretty sharply, but the green or white pictures are very artifacted when moving.
On the top-right there is a drop-down, try all the options in the "Tests" category! Note it has fast-moving imagery that might trigger epilepsy if you're particularly sensitive.
In principle it could be, but usually browsers snap element rectangles to the pixel, thus protecting against that (have you ever seen a blurry border on a box because its contents had 0.5px size? nope, even though it is annoyingly common in things such as game UIs)
Plus you can see the same effect on the UFO tests, which strictly uses integer positions so pixel misalignment is not a factor.
