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chromium/third_party/blink/web_tests/external/wpt/compat/webkit-box-fieldset.html 

<!DOCTYPE html>
<link rel="match" href="webkit-box-fieldset-ref.html">
<p>Fieldset should contain a green div spanning the width.
<fieldset style="display: -webkit-box">
  <div id="child" style="background:green; height:10px"></div>
</fieldset>