docs: def header_check(self, content):
          """Special HTML encoding check."""
  
          encode = None
  
          # Look for meta charset
          m = RE_HTML_ENCODE.search(content)
          if m:
              enc = m.group(1).decode('ascii')
  
              try:
                  codecs.getencoder(enc)
                  encode = enc
              except LookupError:
                  pass
          else:
              encode = self._has_xml_encode(content)
          return encode