Another thing is that they’re saying the animation is causing the entire page to reflow because it’s a square rotating out of bounds

My understanding is that transforms can’t cause reflows because they occur in the compositing layer. And as a sibling comment points out: everything round in css is actually a rectangle. Unless it’s a path drawn in an svg element. But then that lives in a rectangular frame. You could animate it with svg, though.

Regardless, I want to say I reject the premise here. I’m not sure why a transform would be so inefficient. I have a feeling it could be something else at work here.