Event.observe(window, 'load', init, false);
var manipulatedByJsClassName = 'item';

function init()
{
  $$('#dropdown_menu .mainmenu > li').each(function(el) {
    el.observe('mouseover', function() {
      if (!el.hasClassName(manipulatedByJsClassName))
      {
        el.addClassName(manipulatedByJsClassName);
        var submenu = el.down('ul.submenu');
        var pos = el.cumulativeOffset();
        var height = el.getHeight();
        var top = pos.top + height;
        var left = pos.left;
        submenu.setStyle({
          'top'  : top +'px',
          'left' : left +'px'
        });
      }
    });
  });
}