title: get device PPI in javascript
body: <p>How to get real PPI (pixels per inch) device resolution with javascript?</p>
  <p>some examples of device and value expected:</p>
  <ul>
  <li>iMac 27-inch: 109ppi</li>
  <li>iPad: 132ppi</li>
  <li>19inch 1440x900 screen: 89ppi</li>
  <li><a href="https://pixensity.com/list/" rel="noreferrer">...</a></li>
  </ul>