No need to go that far. If you're just looking for the edge of where the capabilities of CSS animation fails, consider the pseudo-physics of various direct manipulations in touch UIs: eg. the bounce when you swipe to scroll and reach the last item, or swipe down again to refresh, or swipe to fling cards aside left or right, or pinch to zoom past the last zoom level, etc.
In some cases you could get a good-enough result by animating from arbitrary (ie. user controlled) starting point to one or more fixed end-states (eg. Three "flick right" animations, one each for flick up and to the right, right, or down and to the right), but at that point you might as well just use an animation library.
To restate things from a bit more mathematical perspective, interpolating position (even with control over easing etc.) only gets you so far. Ideally you would be able to set and interpolate higher derivatives of position over time (speed, acceleration, etc.) as well, without resorting to JS.
In some cases you could get a good-enough result by animating from arbitrary (ie. user controlled) starting point to one or more fixed end-states (eg. Three "flick right" animations, one each for flick up and to the right, right, or down and to the right), but at that point you might as well just use an animation library.