网页尺寸的获取，为何高度很小
<!DOCTYPE HTML>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
<script type="text/javascript">
var w=document.documentElement.offsetWidth
||document.body.offsetWidth;
var h=document.documentElement.offsetHeight
||document.body.offsetHeight;
document.write("网页宽度"+w+"网页高度"+h);
</script>
</body>
</html>
