var elem = "TR";
window.onload = function() {   
  if(document.getElementsByTagName) {   
    var el = document.getElementsByTagName(elem);   
      for(var i=0; i<el.length; i++) {   
        if(el[i].childNodes[0].tagName != "TH"  
          && el[i].parentNode.parentNode.className.indexOf("tbl") != -1) {   
          if(i%2 == 1) {   
            el[i].className = "on";   
          } else {   
            el[i].className = "off";   
          }   
        }   
      }
  } 
}