Responsive Square

<head>
  <style>
    .outer { 
      position: relative;
      width: 25%;
      padding-bottom: 25%;
    }
    .inner {
      position: absolute;
      width: 100%;
      height: 100%;
      background-color: #00FF00; 
    }
  </style>
</head>
<body>
  <div class="outer">
    <div class="inner"></div>
  </div>
</body>