<!DOCTYPE html>
<!--
 | Each block-level element generates a principal block-level box that contains
 | descendant boxes.
 |   https://www.w3.org/TR/CSS21/visuren.html#block-boxes
 -->
<html>
<head>
  <style>
    .red {
      background-color: #f44336;
      height: 100px;
    }
    .purple {
      background-color: #9c27b0;
      height: 100%;
    }
    .green {
      background-color: #4caf50;
      height: 100%;
    }
  </style>
</head>
<body>
  <div class="red">
    <div class="purple">
      <div class="green"></div>
    </div>
  </div>
</body>
</html>