javascript display value in span tag

HTML:

<span id="tag"></span>

JS:

<script type="text/javascript">
   window.onload = function(){
     var text = document.createTextNode("0");
     document.getElementById('tag').appendChild(text);
   }
</script>

Although it would be much easier to just do:

<span id="tag">0</span>

Try:

<script>
  window.onload = function(){
        document.getElementById('tag').innerHTML = 0;
  };
</script>