if(document.getElementById)

function alternate(id){
 if(document.getElementsByTagName){  
   var table = document.getElementById(id);  
   var rows = table.getElementsByTagName("tr");  
   for(i = 0; i < rows.length; i++){          
 //manipulate rows
     if(i % 2 == 0){
       rows[i].className = "even";
     }else{
       rows[i].className = "odd";
     }      
   }
 }
}