innerHTML修改内容,内容是修改了~但是显示不出来!

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<title>0_o</title>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312" />
<style type="text/css">
div{width:98px;height:98px;border:1px solid black;float:left;cursor:pointer;background:red;}
</style>
<script language="javascript">
function getvalue(obj)
{
obj.innerHTML="a";
alert(obj.innerHTML);
}
</script>
</head>
<body>
<div id="ba" onclick="getvalue(this)">xxxxxxxx</div>
</body>
</html>