Snabbt.js is similar to a subset of the concepts in Popmotion; namely Snabbt.js allows one to pick an element anywhere in the DOM then apply matrix-based animation to it with the snabbt function, akin to Popmotion’s ui.Actor. Popmotion has other features beyond that, whereas Snabbt focuses only on that. Both have basic physics abilities.
Someone (one of us, for example) could adopt Snabbt or Popmotion to use as the underlying mechanism for applying matrix transforms to elements in our engine. Our engine is a superset of the concepts in Snabbt or Popmotion, keeping track of an entire scene with a scene graph tree structure, then applying transforms to each node as necessary (Snabbt or Popmotion can be used there).