Both as a noun ("causes jank") and as a verb ("the page janks")!
Unike formal languages, human language doesn't evolve via RFCs or ItIs.
(OK, formal languages don't, really, either. But they do tend to stick stakes in the ground via those mechanisms, for which most human languages don't have any analogous concept.)
Jank is an actual term, it’s the time-derivative of acceleration. Page jank looks how motion jank feels; unexpected and uncomfortable.
Jerk is the derivative of acceleration. I haven't heard jank used in that context.