I'd be very surprised if SVG animations don't use substantially the same rendering paths that CSS animations do. In fact, if anything, I'd expect CSS animations to receive more time and attention from browser devs than the old SMIL elements and attributes, to whatever extent that they aren't equivalent internally.
Yeah but the crux of the problem is when height is animated the browser needs to redo the layout. Here the animation is contained entirely within an <img> tag with known width and height.
