元素和节点不一样：
<div id="d1">
<div id="d2">
<p id="p1">
<p id="p2">
<span id="sp1"></span>
</p>
</p>
</div>
</div>
<span id="sp1">的父节点、父元素都是<p id="p2">
<p id="p2">的父节点是<p id="p1">，父元素是<div id="d2">
<div id="d2">的父节点是<div id="d1">,父元素是<body>
<div id="d1">的父节点、父元素都是<body>