docs: def get_clean_content(self):
          """Implementation of the clean() method."""
          fill_chars = {'BLANK_TEMPLATE': ' ', 'ECHO_TEMPLATE': '0'}
          for match in self.pattern.finditer(self.html_content):
              start, end = match.start(), match.end()
              tag = _get_tag(match)
              if tag == 'ECHO':
                  self._write_content(start)
                  self._index = start
                  self._state = 'ECHO_TEMPLATE'
              elif tag == 'START':
                  if self._index != start:
                      self._write_content(start)
                  self._index = start
                  self._state = 'BLANK_TEMPLATE'
              elif tag == 'END':
                  if self._state not in ('BLANK_TEMPLATE', 'ECHO_TEMPLATE'):
                      # We got a closing tag but none was open. We decide to carry
                      # on as it may be the case that it was because of a closing
                      # dictionary in javascript like: var dict = {foo:{}}.
                      # See the note on the clean() function for more details.
                      continue
                  fill_char = fill_chars[self._state]
                  fill = fill_char * (end - self._index)
                  if self._state == 'BLANK_TEMPLATE':
                      self._pending.append(fill)
                      self._pending_has_blank = True
                  else:
                      assert not self._pending
                      self._output.write(fill)
                  self._index = end
                  self._state = 'HTML'
              elif tag == 'SPACES':
                  self._pending.append(match.group('spaces'))
                  self._index = end
              elif tag == 'NEWLINE':
                  if self._state == 'HTML':
                      if self._index != start or not self._pending_has_blank:
                          self._write_content(start)
                      self._output.write(match.group('newline'))
                  elif self._state == 'BLANK_TEMPLATE':
                      # We discard the content of this template and whatever is in
                      # self._pending.
                      self._output.write(match.group('newline'))
                  elif self._state == 'ECHO_TEMPLATE':
                      assert False, 'Echo tags should be in just one line.'
                  self._index = end
                  self._reset_pending()
  
          assert self._state == 'HTML', 'Tag was not closed'
          if self._index != len(self.html_content) or not self._pending_has_blank:
              self._write_content()
  
          return self._output.getvalue()