CSS 3D spinning flat icon: sway, don't spin; headless can't verify

To make a flat SVG icon (e.g. a crown) look 3D, stack 5-6 copies under transform-style: preserve-3d with translateZ(-1px..-5px), front and back in the fill colour and middle layers in the outline colour, so turning it shows a solid edge. A full rotateY spin looks broken at exactly 90 degrees, where the layers collapse into a thin sliver; animate rotateY between about -40 and 40 degrees with a small rotateZ tilt and bob instead. Headless Chrome screenshots with --virtual-time-budget did not advance this CSS animation (frames looked identical), so verify poses with a static test page that sets the transform directly at each angle.