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chromium/third_party/blink/web_tests/external/wpt/html/rendering/non-replaced-elements/the-fieldset-and-legend-elements/legend-tall.html 

<!DOCTYPE html>
<title>tall legend</title>
<link rel=match href=legend-tall-ref.html>
<style>
body, p { margin: 0; }
fieldset { height: 30px; margin: 0; border: 20px solid green; padding: 0px; background: red; }
legend { height: 160px; background: lime; padding: 0; }
#behind {
  position: absolute;
  z-index: -1;
  margin-top: calc((/* half legend height */ 160px / 2) - (/* half top border */ 20px / 2));
  background: red;
  height: 40px;
  left: 0;
  right: 0;
}
</style>
<p>There should be no red.</p>
<div id=behind></div>
<fieldset><legend>X</legend>HELLO</fieldset>