How can you get the CSS pixel / device pixel ratio?

You can use window.devicePixelRatio in Webkit based browsers to get the device pixel ratio directly in JavaScript. I have used this on Google Chrome, Android browsers (2.2+) and Mobile Safari. I have no idea about other browsers though.

You can always use the following method and it would work in all the browsers

window.getDevicePixelRatio = function () {
    var ratio = 1;
    // To account for zoom, change to use deviceXDPI instead of systemXDPI
    if (window.screen.systemXDPI !== undefined && window.screen.logicalXDPI       !== undefined && window.screen.systemXDPI > window.screen.logicalXDPI) {
        // Only allow for values > 1
        ratio = window.screen.systemXDPI / window.screen.logicalXDPI;
    }
    else if (window.devicePixelRatio !== undefined) {
        ratio = window.devicePixelRatio;
    }
    return ratio;
};

There is a much better CSS/JS solution:

/** sample media query for pixel-ratio=2" **/
@media  
(-webkit-min-device-pixel-ratio: 2), 
(min-resolution: 192dpi) { 
  .pixel-ratio-holder:before {
   content: "2";
  }
}


function jsPixelRatio(){
   return window.getComputedStyle(document.querySelector('.pixel-ratio-holder'), 'before').getPropertyValue('content').replace(/[^a-z]/g,'') * 1;
}

How to get device Pixel Ratio

This example uses window.devicePixelRatio as a starting point AND also window.matchMedia() Web API as a fallback calculation.

The browser support for both those features is pretty good, so this should work great for most of use cases.

Here is a function that retrieves this information, originally written by PatrickJS and published as a GitHub Gist:

function getDevicePixelRatio() {
    var mediaQuery;
    var is_firefox = navigator.userAgent.toLowerCase().indexOf('firefox') > -1;

    if (window.devicePixelRatio !== undefined && !is_firefox) {
        return window.devicePixelRatio;
    } else if (window.matchMedia) {
        mediaQuery = "(-webkit-min-device-pixel-ratio: 1.5),\
          (min--moz-device-pixel-ratio: 1.5),\
          (-o-min-device-pixel-ratio: 3/2),\
          (min-resolution: 1.5dppx)";
        if (window.matchMedia(mediaQuery).matches) {
            return 1.5;
        }
        mediaQuery = "(-webkit-min-device-pixel-ratio: 2),\
          (min--moz-device-pixel-ratio: 2),\
          (-o-min-device-pixel-ratio: 2/1),\
          (min-resolution: 2dppx)";
        if (window.matchMedia(mediaQuery).matches) {
            return 2;
        }
        mediaQuery = "(-webkit-min-device-pixel-ratio: 0.75),\
          (min--moz-device-pixel-ratio: 0.75),\
          (-o-min-device-pixel-ratio: 3/4),\
          (min-resolution: 0.75dppx)";
        if (window.matchMedia(mediaQuery).matches) {
            return 0.7;
        }
    } else {
        return 1;
    }
}

CanIUse: window.devicePixelRatio, Window.matchMedia()

Useful links: MDN - window.devicePixelRatio, MDN - Window.matchMedia()