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chromium/third_party/blink/web_tests/fast/multicol/rule-thicker-than-gap.html 

<!DOCTYPE html>
<html>
  <head>
    <title>Column rule thicker than gap</title>
    <style>
      .mc {
        columns: 2;
        column-gap: 15px;
        column-rule: 31px solid;
        -moz-columns: 2;
        -moz-column-gap: 15px;
        -moz-column-rule: 31px solid;
        -webkit-columns: 2;
        -webkit-column-gap: 15px;
        -webkit-column-rule: 31px solid;
        width: 101px;
      }
    </style>
  </head>
  <body>
    <p>There should be a filled rectangle below.</p>
    <div class="mc">
      <div style="height:200px;"></div>
    </div>
  </body>
</html>