The essence of this technique

web

//pure-CSS drop down menus
div.menu-bar ul ul {
  display: none;
}
div.menu-bar li:hover > ul {
  display: block;
}

//image pre-loader
function preloadImages() {
   if (document.images) {
      for (var i = 0; i < preloadImages.arguments.length; i++) {
         (new Image()).src = preloadImages.arguments[i];
      }
   }
}