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

<!doctype html>
<title>Reference for legend block margins</title>
<style>
body { margin: 0; }
.fieldset { margin: 2em 1em 1em 1em; border: 1em solid green; }
.legend { position: absolute; margin-top: -1em; margin-left: 1em; background: white; height: 1em; }
.inner { margin: 3em 1em 1em 1em; height: 1em; }
</style>
<p>There should be no red.</p>
<div class=fieldset>
  <div class=legend>X</div>
  <div class=inner>Y</div>
</div>