为什么height是0？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
<script type="text/javascript">
var h=document.body.scrollHeight;
var w=document.body.scrollWidth;
document.write(h+"*"+w);
</script>
</body>
</html>
