css/css-contain/contain-inline-size-legend-ref.html

<!DOCTYPE html>
<title>contain:inline-size on legend</title>
<link rel="author" title="Morten Stenshorne" href="mailto:mstensho@chromium.org">
<link rel="help" href="https://drafts.csswg.org/css-contain/#contain-property">
<link rel="help" href="https://drafts.csswg.org/css-contain-3/#containment-inline-size">
<p>There should be blue square (legend), and a yellow square below it (fieldset
  content).</p>
<fieldset style="width:fit-content; min-width:0;">
  <legend style="width:0; border:solid blue; border-width:0 50px; padding:0; background:red;">
    <div style="width:500px; height:100px;"></div>
  </legend>
  <div style="height:100px; background:yellow;"></div>
</fieldset>