怎样在<span>下使用innerhtml?

Question

<span id="span1">
  <table width="100%" height="30" border="1"><tr><td>&nbsp;</td></tr></table>
</span>
<script language="JavaScript">
function addTable(){
var objSpan=document.getElementById("span1");
var shtml=objSpan.innerHTML;
objSpan.innerHTML=shtml+'<table width="100%" height="30" border="1"><tr><td>&nbsp;</td></tr></table>';
}
</script>
<input type="button" value="Add" onClick="addTable();">