Modern browsers only re-render sections of the page as necessary, and don't re-render at all as you scroll (they just move that static image up and down). This

Modern browsers only re-render sections of the page as necessary, and don't re-render at all as you scroll (they just move that static image up and down). This API is for cases where the actual DOM and/or styles change. If that isn't happening, there's no use for it.