retina.js

if (('devicePixelRatio' in window && window.devicePixelRatio >= 1.5) ||
    ('matchMedia' in window && window.matchMedia("(-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)").matches))
{
    document.addEventListener('DOMContentLoaded', function() {
        var elements = document.getElementsByClassName('retina');
        for (var i = 0; i < elements.length; i++) {
            var element = elements[i];
            if (element.tagName !== 'IMG') continue;
            
            var match = element.src.match(/^(.+)\.(.+?)$/);
            if (match) {
                element.src = match[1] + '@2x.' + match[2];
            } else {
                element.src = element.src + '@2x';
            }
            element.onload = function(event) {
                event.target.width = event.target.naturalWidth / 2;
            };
        }
    });
}