<!DOCTYPE html>
<!--
 | If none of the three is "auto": if both "margin-left" and "margin-right" are
 | "auto", solve the equation under the extra constraint that the two margins
 | get equal values, unless this would make them negative, in which case set
 | "margin-left" to zero and solve for "margin-right". If one of "margin-left"
 | or "margin-right" is "auto", solve the equation for that value. If the values
 | are over-constrained, ignore the value for "right" and solve for that value.
 |   https://www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-width
 -->
<html>
<head>
  <style>
    .first {
      top: 10px;
    }
    .second {
      top: 45px;
    }
    .third {
      top: 80px;
    }
    .fourth {
      top: 115px;
    }
    .fifth {
      top: 150px;
    }
    .containing-block {
      background-color: #2196f3;
      height: 5px;
      padding: 10px 0;
      position: absolute;
      width: 500px;
    }
    .block {
      /* TODO: Add borders when supported. */
      background-color: #3f51b5;
      display: block;
      height: 5px;
      left: 50px;
      padding: 0 100px;
      position: absolute;
      right: 50px;
      width: 150px;
    }
    .wide {
      width: 300px;
    }
    .auto-margin-left {
      margin-left: auto;
    }
    .non-auto-margin-left {
      margin-left: 50px;
    }
    .auto-margin-right {
      margin-right: auto;
    }
    .non-auto-margin-right {
      margin-right: 50px;
    }
  </style>
</head>
<body>
  <div class="first containing-block">
    <div class="auto-margin-left auto-margin-right block"></div>
  </div>
  <div class="second containing-block">
    <div class="auto-margin-left auto-margin-right wide block"></div>
  </div>
  <div class="third containing-block">
    <div class="auto-margin-left non-auto-margin-right block"></div>
  </div>
  <div class="fourth containing-block">
    <div class="non-auto-margin-left auto-margin-right block"></div>
  </div>
  <div class="fifth containing-block">
    <div class="non-auto-margin-left non-auto-margin-right block"></div>
  </div>
</body>
</html>