<!DOCTYPE html>
<!--
 | A computed value of "auto" for "margin-left" or "margin-right" becomes
 | a used value of "0".
 |   https://www.w3.org/TR/CSS21/visudet.html#inline-width
 -->
<html>
<head>
  <style>
    .zero-width {
      background-color: #f44336;
    }
    .non-zero-width {
      background-color: #4caf50;
    }
    .container {
      display: inline-block;
      height: 100px;
    }
    .auto-margin {
      margin: 0 auto;
    }
    .non-auto-margin {
      margin: 0 50px;
    }
  </style>
</head>
<body>
  <div class="non-zero-width container">
    <span class="non-auto-margin"></span>
  </div>
  <div class="zero-width container">
    <span class="auto-margin"></span>
  </div>
</body>
</html>