<!DOCTYPE html>
<!--
 | Adjoining vertical margins collapse if both belong to vertically-adjacent box
 | edges, i.e. bottom margin of a last in-flow child and bottom margin of its
 | parent if the parent has "auto" computed height.
 |   https://www.w3.org/TR/CSS21/box.html#collapsing-margins
 -->
<html>
<head>
  <style>
    .outer-green {
      background-color: #00e676;
      margin-bottom: 10px;
    }
    .inner-green {
      background-color: #00c853;
      height: 100px;
      margin-bottom: 100px;
      width: 100px;
    }
    .outer-blue {
      background-color: #03a9f4;
      height: 100px;
      width: 100px;
    }
  </style>
</head>
<body>
  <div class="outer-green">
    <div class="inner-green"></div>
  </div>
  <div class="outer-blue"></div>
</body>
</html>