<p><u>underlined text</u></p>

__underlined text__

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE document SYSTEM "CommonMark.dtd">
<document xmlns="http://commonmark.org/xml/1.0">
  <paragraph>
    <underline>
      <text xml:space="preserve">underlined text</text>
    </underline>
  </paragraph>
</document>

<p><em><u>this</u></em></p>

*__this__*

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE document SYSTEM "CommonMark.dtd">
<document xmlns="http://commonmark.org/xml/1.0">
  <paragraph>
    <emph>
      <underline>
        <text xml:space="preserve">this</text>
      </underline>
    </emph>
  </paragraph>
</document>