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()