<!DOCTYPE html>
<!--
 | If both "margin-left" and "margin-right" are "auto", their used values
 | are equal. This horizontally centers the element with respect to the edges
 | of the containing block.
 |   https://www.w3.org/TR/CSS21/visudet.html#blockwidth
 -->
<html>
<head>
  <style>
    .containing-block {
      background-color: #2196f3;
      padding: 10px 0;
      margin-bottom: 10px;
      width: 500px;
    }
    .block {
      /* TODO: Add borders when supported. */
      background-color: #3f51b5;
      display: block;
      height: 5px;
      margin: 0 auto;
      padding: 0 100px;
      width: 100px;
    }
  </style>
</head>
<body>
  <div class="containing-block">
    <div class="block"></div>
  </div>
</body>
</html>