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chromium/third_party/blink/web_tests/printing/thead-repeat-in-nested-table.html 

<!DOCTYPE html>
<style>
table { border-spacing: 0; }
th { height: 100px; }
td td { height: 550px; padding: 0; }
</style>
<script>
if (window.testRunner)
  testRunner.setPrinting();
</script>
<table>
  <thead>
    <tr><th>First level header</th></tr>
  </thead>
  <tr>
    <td>
      <table>
        <thead>
          <tr><th>Second level header</th></tr>
        </thead>
        <tr><td>Body</td></tr>
        <tr><td>Body</td></tr>
        <tr><td>Body</td></tr>
      </table>
    </td>
  </tr>
</table>