老师帮我看看代码是正确的吗？

来源：2-4 编程练习

慕妹8524794

2019-10-15 21:27:26

<!DOCTYPE html>
<html>
<head>
    <meta charset="utf-8">
<title>CSS布局</title>
<style type="text/css">
  *{
   margin: 0;
   padding:0;
  }
  img{
   width: 380px;
   height: 240px;
  }
  /*此处写代码*/
  .mask{
      width:380px;
      height:240px;
      background:#000;
      opacity:0.5;
      position:absolute;
      left:0;
      top:0;
      z-index:2;
  }
</style>
</head>
<body>
<nav>
</nav>
<div>
  <img src="http://climg.mukewang.com/58f829ca00010be703840240.jpg">
</div>

<div class="mask"></div>
</body>
</html>