SOURCE

body:after {
    content: "1x / 96 dpi";
}
@media (-webkit-min-device-pixel-ratio: 1.5), (min-resolution: 120dpi) {
  body:after {
    content: '1.5x / 120dpi';
  }
}
@media (-webkit-min-device-pixel-ratio: 2), (min-resolution: 192dpi) {
  body:after {
    content: '2x / 192 dpi';
  }
}
@media (-webkit-min-device-pixel-ratio: 3), (min-resolution: 288dpi) {
  body:after {
    content: '3x / 288 dpi';
  }
}
@media (-webkit-min-device-pixel-ratio: 4), (min-resolution: 384dpi) {
  body:after {
    content: '4x / 384 dpi';
  }
}