两个高怎么是这样，大佬能解释下吗

<script type="text/javascript">

    var h=document.body.offsetHeight;

    document.write(h+"<br>");//结果为0

    var w=document.body.offsetWidth;

    document.write(w+"<br>"); 

    var w= document.documentElement.offsetWidth;

    document.write(w+"<br>");

var h= document.documentElement.offsetHeight;

    document.write(h);//结果为79

    </script>