<!DOCTYPE html>
<!--
 | Adjoining vertical margins collapse if both belong to vertically-adjacent box
 | edges, i.e. bottom margin of box and top margin of its next in-flow following
 | sibling.
 |   https://www.w3.org/TR/CSS21/box.html#collapsing-margins
 -->
<html>
<head>
  <style>
    .blue {
      background-color: #2196f3;
      height: 100px;
      margin-bottom: 50px;
      width: 100px;
    }
    .indigo {
      background-color: #3f51b5;
      height: 100px;
      margin-top: 50px;
      width: 100px;
    }
  </style>
</head>
<body>
  <div class="blue"></div><div class="indigo"></div>
</body>
</html>