var saveClassName;

mouseHighlight = function() {
  var i, tr = document.getElementsByTagName( 'tr' );
  
  for(i = 0; i < tr.length; i++) {
    if ( tr[i].className == 'odd' || tr[i].className == 'even' ) {
      tr[i].onmouseover = function() { 
      	saveClassName = this.className;
      	this.className = 'tr_hover';
       };
      tr[i].onmouseout = function() { this.className = saveClassName };
    }
  }
}
window.onload = mouseHighlight;