task 3：编写函数实现单击change按钮，为div元素添加红色双线的边框。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>编写函数实现单击change按钮，为div元素添加红色双线的边框。</title>
<style type="text/css">
div{
font-family: "Helvetica Neue",Helvetica,Arial,"Microsoft Yahei",sans-serif;
width: 600px;
text-align: center;
}
.haha{
margin-top: 20px;
margin-left: 250px;
font-size: 20px;
}
.haha:hover{
border: 3px solid purple;
}
</style>
</head>
<body>
<div id="joy"> <p>123456。</p>
</div>
<button class="haha" onclick="myFunction()">change</button>
<script type="text/javascript">
function myFunction(){
var Color = document.getElementById("joy");
Color.style.border = "3px double red";
Color.innerHTML = "<p>123456789</p>";
}
</script>
</body>
</html>
