<!DOCTYPE html>
<!--
 | If the inline box contains no glyphs at all, it is considered to contain
 | a strut (an invisible glyph of zero width) with the A (ascent) and
 | D (descent) of the element's first available font.
 |   https://www.w3.org/TR/CSS21/visudet.html#leading
 -->
<html>
<head>
  <style>
    .a {
      background-color: #1de9b6;
      font-size: 100px;
    }
    .b {
      background-color: #00bfa5;
      display: inline-block;
      height: 5px;
      width: 100px;
    }
  </style>
</head>
<body>
  <div class="a"><div class="b"></div></div>
</body>
</html>