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// based on article by CHRIS COYIER https://css-tricks.com/responsive-images-cs
.img {
  background-image: url(small.jpg);
  background-image: 
    -webkit-image-set(
      "small.jpg" 1x,
      "medium.jpg" 2x,
    );
  background-image: 
    image-set(
      "small.jpg" 1x,
      "medium.jpg" 2x,
    );
}
@media
  (min-width: 800px) {
  .img {
    background-image: url(large.jpg);
    background-image: 
      -webkit-image-set(
        "large.jpg" 1x,
        "extralarge.jpg" 2x,
      );
    background-image: 
      image-set(
        "large.jpg" 1x,
        "extralarge.jpg" 2x,
      );
  }
}