Python Markdown の html フォーマットを変更する

Bad value “footnote” for attribute “rel” on element “a”: The string “footnote” is not a registered keyword.  html-checker

class FootnotePattern(Pattern):
    """ InlinePattern for footnote markers in a document's body text. """

    def __init__(self, pattern, footnotes):
        super(FootnotePattern, self).__init__(pattern)
        self.footnotes = footnotes

    def handleMatch(self, m):
        id = m.group(2)
        if id in self.footnotes.footnotes.keys():
            sup = util.etree.Element("sup")
            a = util.etree.SubElement(sup, "a")
            sup.set('id', self.footnotes.makeFootnoteRefId(id, found=True))
            a.set('href', '#' + self.footnotes.makeFootnoteId(id))
            if self.footnotes.md.output_format not in ['html5', 'xhtml5']:
                a.set('rel', 'footnote')  # invalid in HTML5
            a.set('class', 'footnote-ref')
            a.text = util.text_type(self.footnotes.footnotes.index(id) + 1)
            return sup
        else:
            return None

def codehilite(content):
    """
    Renders content using markdown with the codehilite extension.
    """
    return _clean(markdown(content, ['codehilite',]), output_format="html5")