<html>
<head>
<script type="text/javascript">
function getInnerHTML() {
document.getElementById("tr3").innerHTML="<td>aaaa</td><td>ttttt</td>";
alert(document.getElementById("tr3").innerHTML);
}
</script>
</head>
<body>
<table border="1">
<tr id="tr1"><td>Firstname</td><td>Lastname</td></tr>
<tr id="tr2"><td>Peter</td><td>Griffin</td></tr>
<tr id="tr3"></tr>
</table>
<br />
<input type="button" onclick="getInnerHTML()" value="test" />
</body>
</html>