Add class "retina" in HTML tag for high pixel ratio devices

isRetina.js

// jQuery mode
if (window.devicePixelRatio > 1) {
  $('body').addClass('retina');
}
 
// Only javascript mode
if (window.devicePixelRatio > 1) {
  var elHtml = document.getElementsByTagName('html')[0];
  var atrClass = elHtml.getAttribute('class');
      atrClass = atrClass + ' retina';
  
  elHtml.setAttribute('class', atrClass); //For Most Browsers
  //elHtml.setAttribute('className', atrClass); //For IE; harmless to other brow
}