<!DOCTYPE html>
<!--
 | The "em" unit should be relative to the computed font size of self, hence
 | relative to parent.
 |   https://www.w3.org/TR/css3-values/#font-relative-lengths
 -->
<html>
<head>
  <style>
    body {
      font-family: roboto;
    }
    div {
      font-size: 1.5em;
    }
  </style>
</head>

<body>

<div>
  outer div
  <div>
    middle div
    <div>
      inner div
    </div>
  </div>
</div>

</body>
</html>