preload_images.js

(function () {
  'use strict';

  var i, image, j, matches, rules, sheet;

  for (i = 0; i < document.styleSheets.length; ++i) {
    sheet = document.styleSheets[i];
    if (sheet.rules) {
      for (j = 0; j < sheet.rules.length; ++j) {
        rules = sheet.rules[j];
        if (rules.style && rules.style.backgroundImage) {
          matches = rules.style.backgroundImage.match(/url\((.*)\)/);
          if (matches) {
            image = new Image;
            image.src = matches[1];
          }
        }
      }
    }
  }

})();