It’s explained in the article: the image is a square (even though it looks like a circle), rotating a square cannot be done in place, it’ll shift the layout as it rotates, provoking a new paint of the entire page on each animation frame.
This is how the author explains it, but it sounds suspicious to me, I’m not sure a CSS animate rotation would cause shifts in that case.
This is how the author explains it, but it sounds suspicious to me, I’m not sure a CSS animate rotation would cause shifts in that case.