document.head がいつも同じ値である保証は無い 

<html>
<head id=headelement>
<title>document.head></title>
<script>
var id1=document.head.id;
</script>
</head>
<body>
<script>
var id2 = document.head.id;
</script>
<form name=head id=formelement><input type=text></form>
<script>
var id3 = document.head.id;
document.write(id1);
document.write("<br>");
document.write(id2);
document.write("<br>");
document.write(id3); //id3 is "formelement" on some browser.
</script>
</body>
</html>