<!DOCTYPE html>
<!--
 | If "width" is not "auto" and "border-left-width" + "padding-left" + "width" +
 | "padding-right" + "border-right-width" (plus any of "margin-left" or
 | "margin-right" that are not "auto") is larger than the width of
 | the containing block, then any "auto" values for "margin-left" or
 | "margin-right" are treated as zero.
 |   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;
    }
    .non-auto-width-and-auto-margin-left {
      margin-left: auto;
      width: 150px;
      margin-right: 200px;
    }
    .non-auto-width-and-auto-margin-right {
      margin-left: 200px;
      width: 150px;
      margin-right: auto;
    }
    .non-auto-width-and-auto-margin-left-and-right {
      margin-left: auto;
      width: 350px;
      margin-right: auto;
    }
  </style>
</head>
<body>
  <div class="containing-block">
    <div class="non-auto-width-and-auto-margin-left block"></div>
  </div>
  <div class="containing-block">
    <div class="non-auto-width-and-auto-margin-right block"></div>
  </div>
  <div class="containing-block">
    <div class="non-auto-width-and-auto-margin-left-and-right block"></div>
  </div>
</body>
</html>