<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://apps.bdimg.com/libs/bootstrap/3.3.0/css/bootstrap.min.css">
<script src="https://libs.baidu.com/jquery/2.1.4/jquery.min.js"></script>
<script src="https://apps.bdimg.com/libs/bootstrap/3.3.0/js/bootstrap.min.js"></script>
<style>
.collapsing { background-color: yellow;
}
</style>
</head>
<body>
<div class="container">
<h2>Simple Collapsible</h2>
<button type="button" class="btn btn-lg btn-info collapsed" data-toggle="collapse" data-target="#demo">Simple collapsible</button>
<div id="demo" class="collapse">
The .collapsing class is added when the transition starts, and removed when it is finished. In this example, the collapsible content gets a yellow background for those seconds it takes to show the content. The background color is removed when the transition has ended.
</div>
</div>
</body>
</html>