请问怎么控制背景图片居中在题目要求的位置

来源：2-15 编程练习

慕粉085629689

2020-03-25 11:33:36

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>background属性</title>
    <style type="text/css">
        /*此处写代码*/
    header{
        width:500px;
        height:300px;
        border:20px solid rgba(255,0,0,0.5);
        background:url(http://climg.mukewang.com/582c39c00001091605000150.jpg) no-repeat;
        background-size:90%;
        background-clip:padding-box;
        background-color:#999;
         margin:0 auto;

    }
    .a{
        font-weight:bold;
        text-align:center;
        padding-top:20px;
    }
    .b{
        padding:20px;
        padding:20px;
    }
    </style>
</head>
<body>
    
    <header>
        <div class="a">慕课网(IMOOC)国内最大的IT技能学习平台</div>
        <div class="b"></div>
    </header>
</body>
</html>