为什么这三种写法得到的答案不同

源自：9-20 网页尺寸offsetHeight

<!DOCTYPE HTML>

<head>

</head>

<body>

var w=document.documentElement.offsetWidth;

var h=document.documentElement.offsetHeight;

document.write(w+"***"+h+"<br>");

document.write(document.body.offsetWidth+"<br>");

document.write(document.body.offsetHeight+"<br>");

var x = document.documentElement.offsetWidth || document.body.offsetWidth;

var y= document.documentElement.offsetHeight || document.body.offsetHeight;

document.write("宽度：" + x);

document.write("<br>高度：" + y);

</script>

</body>

</html>

提问者：黑豆0 2016-02-23 11:37