Executing JavaScript every frame is not that heavy, but if the main thread is blocked because of something that is, which may even be in another tab, the animation will stutter.
The animations built into the platform can run and remain smooth under a broader range of conditions, which is why they should be preferred if you don't need the level of control that running script via requestAnimationFrame gives you.
The animations built into the platform can run and remain smooth under a broader range of conditions, which is why they should be preferred if you don't need the level of control that running script via requestAnimationFrame gives you.