<!DOCTYPE html>
<!--
 | If all of properties have a computed value other than "auto", the values are
 | said to be "over-constrained" and one of the used values will have to be
 | different from its computed value. If the "direction" property of the
 | containing block has the value "ltr", the specified value of "margin-right"
 | is ignored and the value is calculated so as to make the equality true. If
 | the value of "direction" is "rtl", this happens to "margin-left" instead.
 |   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;
      padding: 0 100px;
      width: 100px;
    }
    .margin-left {
      margin-left: 50px;
    }
    .margin-right {
      margin-right: 50px;
    }
  </style>
</head>
<body>
  <div class="containing-block">
    <div class="margin-left block"></div>
  </div>
  <div class="containing-block">
    <div class="margin-right block"></div>
  </div>
  <div class="containing-block">
    <div class="margin-left margin-right block"></div>
  </div>
</body>
</html>