function alternate(){
   if(document.getElementsByTagName) {  
     var table = document.getElementsByTagName("table");  
     for (i = 0; i < table.length; i++) {
       var rows = table[i].rows;
       for(j = 0; j < rows.length; j++) {          
         //manipulate rows
         if(j % 2 == 1){
           rows[j].className = "white";
         } else {
           rows[j].className = "dark";
         }      
       }
     }
   }
}