function horizontal() {
   var navItems = document.getElementById("menu-principal").getElementsByTagName("li");
   
   for (var i=0; i< navItems.length; i++) {
          if(navItems[i].getElementsByTagName('ul')[0] != null)
         {
            navItems[i].onmouseover=function() { this.getElementsByTagName('ul')[0].style.display="block";}
            navItems[i].onmouseout=function() { this.getElementsByTagName('ul')[0].style.display="none"; }
         }
   }
 
}