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;

}

Hi @DCWizard thanks for contribution I'll add your snippet to gist

Thanks,
If you indulge me a little bit further.
It does not make sense to me to have only A DPI.
That doesn't make sense in a first place.
You have a width and a height. Not all screen have the same shape and proportions.
So, you should have a X DPI and a Y DPI.
One monitor I have is 1280x1024 on a 13x11in. Everything look weird on that monitor.
i.e. Movie/Video especially.

Strange that on printers and scanners (the good one) have all that info.
I should know. I made some of those.
As for 3D printers you also need a Z DPI.
Otherwise, you cannot phantom to produce anything worthy of the real world.

I think, I have just give my whole input on this now.
Let me know if I missed anything.