That's 1) just for CSS, 2) incompatible with border-image and list-style-image properties and 3) horribly inefficient, with ugly syntax and a bunch of duplicated effort for developers (since filenames for 1x and 2x image variants are typically identical except for a few extra characters used site-wide for all 2x versions, there should be a way to allow developers to specify only these unique characters one time and avoid that extra work when possible).