motion-path is specced and already has some support. But there is another way to replicate curved motion paths, as Tobias Ahlin points out:

… if we add a container around the object we want to animate, we can apply one timing function for the X-axis, and another one for the Y-axis. Below, we’re using ease-in for the X-axis, and ease-out for the Y-axis.

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Moving Along a Curved Path in CSS is a post from CSS-Tricks