Use img { image-rendering: pixelated }

Chrome automatically applies smoothing when drawing images to screen when in retina mode while the image was captured non-retina.

Since backstop's goal is to compare images, the smoothing is counterproductive. I am not seeing what the browser actually rendered - I get a blurred version instead.

The `img { image-rendering: pixelated }` style makes the image comparison render much crisper.

I am currently applying this style in DevTools, but I figure this could be done automatically by backstop.js.