html

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

css

/* Exact resolution */
@media (resolution: 150dpi) {
  p {
    color: red;
  }
}

/* Minimum resolution */
@media (min-resolution: 72dpi) {
  p {
    text-decoration: underline;
  }
}

/* Maximum resolution */
@media (max-resolution: 300dpi) {
  p {
    background: yellow;
  }
}