Motion Path is a CSS module that allows authors to animate any graphical object along a custom path.

The idea is that when you want to animate an element moving along a path, you previously only had animating translation, position, etc. at your disposal, which wasn't ideal and only allowed for simple movements. With offset-path you can define a specific path of any shape you want. You then animate it along that path by animating offset-distance, and can choose to rotate it at any point using offset-rotate.