请教一下那个border:hover div?谢谢！

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>background-clip</title>
    <style type="text/css">
        .border,.padding,.content{/*给最外层的div设置样式*/
            width:300px;height:300px;
            float:left;margin-left:50px;
        }
        a{
            text-decoration:none;
            font-size:30px;
        }
        .div1,.div2,.div3{/*放置背景图片的div样式*/
            width:220px;height:200px;
            border:10px solid rgba(0,255,0,0.3);
            padding:50px; 
            background-image:url("http://img1.sycdn.imooc.com/climg//582c316e0001fd6507000210.jpg");
            margin-top:50px;
            display: none;
        }
        .border:hover div,.padding:hover div,.content:hover div{
            display:block;
        }
        /*补充代码*/
        a:hover + .div1{
            display: block;
            background-clip: border-box;
        }
        a:hover + .div2{
            display: block;
            background-clip: padding-box;
        }
        a:hover + .div3{
            display: block;
            background-clip: content-box;
        }

    </style>
</head>
<body>
    <div class="border">
        <a href="">border-box</a>                   
        <div class="div1"></div>            
    </div>
    <div class="padding">
        <a href="">padding-box</a>
        <div class="div2"></div>
    </div>
    <div class="content">
        <a href="">content-box</a>             
        <div class="div3"></div>
    </div>
</body>
</html>

慕九州4495990 2019-06-19

源自：CSS3背景与渐变 2-4 编程练习