<!DOCTYPE html>
<!--
 | Bottom margin of a last in-flow child and bottom margin of its parent margins
 | are not adjoining if the parent has computed height other than "auto",
 | and thus they do not collapse.
 |   https://www.w3.org/TR/CSS21/box.html#collapsing-margins
 -->
<html>
<head>
  <style>
    .outer-green {
      background-color: #00e676;
      margin-bottom: 10px;
      height: 100px;  /* This line prevents margin collapsing. */
    }
    .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>