Getting the native DPI

gistfile1.txt

function getDPI() {
  var native = document.createElement('div');
  native.setAttribute('style', 'width:0px; width:100mozmm; position: fixed;');
  document.body.appendChild(native);
  var standard = document.createElement('div');
  standard.setAttribute('style', 'width:100mm; position: fixed;');
  document.body.appendChild(standard);
  var dpi = Math.round(native.offsetWidth / standard.offsetWidth * 96);
  native.remove();
  standard.remove();
  return dpi;
}