<!DOCTYPE html>

<html>
<head>
    <style type="text/css">
      .content {
        background-color: silver;
        height: 200px;
        margin: 5px;
      }

      .fixed {
        position: fixed;
        top: 0;
        left: 0;
        bottom: 0;
        right: 0;
        margin: 5px;
        background-color: rgba(0, 0, 0, 0.8);
      }
      
      .fixed > .contents {
        background-color: white;
        width: 80%;
        height: 500px;
        margin: 100px auto;
      }
    </style>
</head>
<body>

  <div class="content"></div>
  <div class="content"></div>

  <div class="fixed">
    <div class="contents">
      Contents
    </div>
  </div>

  <div class="content"></div>
  <div class="content"></div>

</body>
</html>
