Set document meta on retina display

setRetinaMeta.js

var docEl = document.documentElement;
var metaEl = document.createElement('meta');
var scale = devicePixelRatio > 1 ? 0.5 : 1;
metaEl.setAttribute('name', 'viewport');
metaEl.setAttribute('content', 'initial-scale=' + scale + ', maximum-scale=' + scale + ', minimum-scale=' + scale + ', user-scalable=no');
if (docEl.firstElementChild) {
  docEl.firstElementChild.appendChild(metaEl);
} else {
  var wrap = document.createElement('div');
  wrap.appendChild(metaEl);
  document.write(wrap.innerHTML);
}