<!DOCTYPE html>
<!--
 | Block-level boxes are boxes that participate in a block formatting context.
 |   https://www.w3.org/TR/CSS21/visuren.html#block-boxes
 -->
<html>
<head>
  <style>
    .lighter-blue {
      background-color: #42a5f5;
      height: 100px;
      width: 100px;
    }
    .darker-blue {
      background-color: #2196f3;
      height: 100px;
      width: 100px;
    }
  </style>
</head>
<body>
  <div class="lighter-blue"></div>
  <div class="darker-blue"></div>
</body>
</html>