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";
          }
        }
      }
  }      
}