求大侠指点，为什么以下代码 getElementById获取不到 id='myshow'的div？
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<script language="javascript" type="text/javascript">
var el=document.getElementById("myshow");
el.innerHTML="dfsdfdf"
</script>
</head>
<body>
<div id="myshow"></div>
</body>
</html>
