<!DOCTYPE html>
<!--
 | Anonymous block boxes are ignored when resolving percentage values
 | that would refer to it: the closest non-anonymous ancestor box is used
 | instead.
 |   https://www.w3.org/TR/CSS21/visuren.html#anonymous-block-level
 -->
<html>
<head>
  <style>
    .outer {
      background-color: #2196f3;
      height: 200px;
      width: 200px;
    }
    .inner {
      background-color: #3f51b5;
      display: inline-block;
      height: 50%;
      width: 50%;
    }
  </style>
</head>
<body>
  <div class="outer">
    <span class="inner"></span>
    <div></div>
  </div>
</body>
</html>