<!DOCTYPE html>
<html>

<head>
    <meta charset="UTF-8">
    <style>
       .box {
    width: 100px;
    height: 100px;
    /*补全代码*/
    position:fixed;
    top:0;
    left:0;
    background-color:yellow;
}
    </style>
</head>

<body>
  <div class='box'></div> 
    <script type="text/javascript">
        /* 填写JavaScript */
        
    </script>
</body>

</html>