var elem = "TR";
if (window.addEventListener) {
    window.addEventListener('load', function() {
          if(document.getElementsByTagName) {
    var el = document.getElementsByTagName(elem);
      for(var i=0; i<el.length; i++) {
         {
          if(i%2 == 1) {
            el[i].className = "on";
          } else {
            el[i].className = "off";
          }
        }
      }
  }
    }, false);
}else if (window.attachEvent) {
window.attachEvent('onload', function() {
          if(document.getElementsByTagName) {
    var el = document.getElementsByTagName(elem);
      for(var i=0; i<el.length; i++) {
          if(i%2 == 1) {
            el[i].className = "on";
          } else {   
            el[i].className = "off";
          }
        }
      }
  });
}