-webkit-device-pixel-ratio

/* A unit of "dppx" is implied: */
@media (-webkit-min-device-pixel-ratio: 2) {
  /* … */
}
/* It is equivalent to: */
@media (min-resolution: 2dppx) {
  /* … */
}
/* Similarly: */
@media (-webkit-max-device-pixel-ratio: 2) {
  /* … */
}
/* It is equivalent to: */
@media (max-resolution: 2dppx) {
  /* … */
}

<p>This is a test of your device's pixel density.</p>

/* Exact resolution */
@media (-webkit-device-pixel-ratio: 1) {
  p {
    color: red;
  }
}
/* Minimum resolution */
@media (-webkit-min-device-pixel-ratio: 1.1) {
  p {
    font-size: 1.5em;
  }
}
/* Maximum resolution */
@media (-webkit-max-device-pixel-ratio: 3) {
  p {
    background: yellow;
  }
}