Get DPI from javascript (live at: https://jsfiddle.net/bsorrentino/s8c51jvt/)

getDPI.js

//
// inspired by https://stackoverflow.com/a/838755/521197
//
function getDPI() {
    var div = document.createElement( "div");
    div.style.height = "1in";
    div.style.width = "1in";
    div.style.top = "-100%";
    div.style.left = "-100%";
    div.style.position = "absolute";

    document.body.appendChild(div);

    var result =  div.offsetHeight;

    document.body.removeChild( div );

    return result;

}