Because these animations only work in rigidly specified isolated static objects. Because god forbid any animation touches layout.

Well, many of those animations like opacity changes and border animations undoubtedly force the browser to re-layout and repaint the entire screen anyway. So on an actual page you'll se stutters, frame drops and ungodly CPU usage.

The only animations that will (barely) work is the `transform: translate` ones as they usually preserve the layout, and can be composited on the GPU.