Fetch each CSS background-image cropped & scaled to exactly the
pixels the browser paints.
Each pair shows the same element twice. The
original loads the full source image from
dummyimage.com; the optimized one is rewritten by this
library to fetch only the visible region through the
wsrv.nl image proxy. They render identically — but the optimized
request is smaller. Every value below is measured live at your device's pixel
ratio (?×).