实例1：一串跟着鼠标飞舞的div
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style>
div {width:10px; height:10px; background:red; position:absolute;}
</style>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>一串跟着鼠标飞舞的div </title>
<script>
window.onload=function ()
{
var aDiv=document.getElementsByTagName(‘div‘);
var i=0;
document.onmousemove=function (ev)
{
var oEvent=ev||event; //兼容浏览器的事件对象的写法，event是IE的写法，传参是火狐的写法
for(i=aDiv.length-1;i>0;i--)
{
aDiv[i].style.left=aDiv[i-1].style.left;
aDiv[i].style.top=aDiv[i-1].style.top;
}
aDiv[0].style.left=oEvent.clientX+‘px‘;
aDiv[0].style.top=oEvent.clientY+‘px‘;
};
};
</script>
</head>
<body>
<div></div>
<div></div>
<div></div>
<!--------------此处省略很多div-------->
</body>
</html>
2.document是<!DOCTYPE 和html标签的父级
原文：http://www.cnblogs.com/lanyueff/p/4604104.html