<level_1>
  <level_2a>
    <level_3a>...
    </level_3a>
    <level_3b>...
    </level_3b>
  </level_2a>
  <level_2b>
    <level_3c>...
    </level_3c>
    <level_3d>...
    </level_3d>
  </level_2b>
</level_1>

var p1 = document.getElementsByTagName('level_1');
p1.onchange = function {
  alert('Element Changed');
}