目前知道有两种方式：
1、

<html>
<head>
	<title>onload test</title>
	<script language="javascript">
	window.onload = function(){
		alert(document.getElementById("myID"));
	}
	</script>
</head>
<body>
<p id="myID">测试内容</p>
</body>
</html>

2、

<html>
<head>
	<title>onload test</title>
	<script language="javascript">
	function test(){
		alert(document.getElementById("myID"));
	}
	</script>
</head>
<body onload="test();">
<p id="myID">测试内容</p>
</body>
</html>

而以下方式则不可行：

<html>
<head>
	<title>onload test</title>
	<script language="javascript">
	function test(){
		alert(document.getElementById("myID"));
	}
	window.onload = test();
	</script>
</head>
<body>
<p id="myID">测试内容</p>
</body>
</html>