xywh.js. xywh.js is a JavaScript polyfill that lets you crop images and videos simply by using specific x, y, width, and height information from their URIs (see mark-up examples below).The library implements the spatial media fragments dimension of the W3C Media Fragments URI specification as a polyfill. Please refer to http://www.w3.org/TR/media-frags/#naming-space for the full details.