function defaultMenu()
{
	level0 = $('nav').childElements();
	active = false;
	level0.each(function(li) {
		if (li.hasClassName('active')) {
			active = true;
		}
	});
	if (!active) {
		$('nav').firstDescendant().addClassName('active');
	}
}

Event.observe(window, 'load', function(){
   defaultMenu();
});