javascript可以判断元素是否位于其他元素之上?
考虑这个标记:
<!doctype html>
<html>
<style>
section div {width: 100px; position: absolute; border: 1px solid black; }
</style>
<body>
<section>
<div id="div1" style="height: 400px; background: blue;"></div>
<div id="div2" style="height: 300px; background: red;"></div>
<div id="div3" style="height: 200px; background: yellow;"></div>
<div id="div4" style="height: 100px; background: green;"></div>
</section>
</body>
</html>
我怎么知道(例如)div3下是否有什么东西?