CSS：元素的显示模式、背景、三大特性

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<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        a {
            display: block;
            color: black;
            background-color: dimgrey;
            line-height: 40px;
            text-decoration: none;
            text-align: center;
            font-size: 14px;
            height: 40px;
            width: 230px;
        }
        
        a:hover {
            background-color: orange;
        }
        
        .div1 {
            width: 200px;
            height: 200px;
            background-color: aqua;
        }
        
        .div2 {
            width: 600px;
            height: 600px;
            background-color: bisque;
            background-image: url("../images/yang\'s帅照.jpg");
            /* repeat-x -y */
            background-repeat: no-repeat;
            background-position: center top;
        }
        
        .div3 {
            width: 600px;
            height: 600px;
            background-color: rgb(0, 0, 0, 0.6);
            background-image: url("../images/yang\'s帅照.jpg");
            background-position: center top;
            background-attachment: fixed;
        }
    </style>
</head>

<body>
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    <a href="http://www.whu.edu.cn" target="_blank">武汉大学</a>
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    <div class="div2">
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    <div class="div3">
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