为什么高输出为0?
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
</head>
<body>
<script type="text/javascript">
document.write("当前窗口宽:"+document.body.clientWidth+"<br />"+"当前窗口高:"+document.body.clientHeight);
</script>
</body>
</html>