js中事件三阶段

先贴代码：
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>冒泡</title>
    <style>
        #a{
            width: 300px;
            height:300px;
            background-color: cadetblue;
        }
        #b{
            width: 200px;
            height:200px;
            background-color: chocolate;
        }
        #c{
            width: 100px;
            height:100px;
            background-color: coral;
        }
    </style>
</head>
<body>
<div id="a">
    <div id="b">
        <div id="c"></div>
    </div>
</div>
<script src="mainx.js"></script>
</body>
</html>
html