Usually border-radius: 50% works fine for most applications, and Chrome produces what looks like a circle. But in this instance, I am trying to continually rota
It's caused by the roundings in the way the "radius" is calculated. Since the size is an even number the border is "in-between" two pixels... long history, at the end:
Workarround: Set your divs circles size an "odd" number of pixels.