Bootstrap 过渡效果

<!DOCTYPE html>
<html <!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>下拉菜单</title>
    <link href="css/bootstrap.min.css" rel="stylesheet" type=text/css"/>
    
    <script type="text/javascript" src="js/jquery-3.0.0.min.js"></script>
    <script type="text/javascript" src="js/bootstrap.min.js"></script>
    <style>
        #mydiv{
            200px;
            height:200px ;
            background:seagreen ;
            transition:height 4s;
            -webkit-transition:height  4s;
        }
        #mydiv:hover {
        height:400px ;
        }
    </style>
</head>
<body>
<div id="mydiv">
    
</div>
<script>
    document.getElementById("mydiv").addEventListener("transitionend",function(e){
        this.innerHTML="动画效果";
        
    })
</script>
</body>
</html>
效果：