<!DOCTYPE html>
<html>
  <head>
    <title>Column rule thicker than gap</title>
    <style>
      .mc {
        border-left: 31px solid;
        margin-left: 35px;
        height: 100px;
      }
    </style>
  </head>
  <body>
    <p>There should be a filled rectangle below.</p>
    <div class="mc"></div>
  </body>
</html>