I don't know if it's "on purpose", but that's a correct result of how the animations are defined in the CSS, not a browser bug. The stylesheet defines a single

I don't know if it's "on purpose", but that's a correct result of how the animations are defined in the CSS, not a browser bug. The stylesheet defines a single set of animation keyframes going from top to bottom at constant speed with a small bounce, and then applies the default CSS "ease" timing function to the entire animation. That results in an incorrect non-zero vertical initial velocity (making it seem like the ball was thrown downward rather than dropped), an initial downward acceleration that's vaguely in the right ballpark, and then a deceleration that's completely incorrect.