background-set.css

.image {
  background: set('/images/icons/baz.jpg') at-4x;
}

/* ------ ------ ------ ------ ------ ------ ------ ------ ------ ------ */

.image {
  background: url('/images/icons/baz@1x.jpg');
}

@media (min-device-pixel-ratio: 1.5), (min-resolution: 144dpi), (min-resolution: 1.5dppx) {
  .image {
    background: url('/images/icons/baz@2x.jpg');
  }
}

@media (min-device-pixel-ratio: 2.5), (min-resolution: 216dpi), (min-resolution: 2.5dppx) {
  .image {
    background: url('/images/icons/baz@3x.jpg');
  }
}

@media (min-device-pixel-ratio: 3.5), (min-resolution: 288dpi), (min-resolution: 3.5dppx) {
  .image {
    background: url('/images/icons/baz@4x.jpg');
  }
}