Improve the robustness of the HTML parser

        self.tag_required_blocklist = required_blocklisted_tags

        self.tag_blocklist = blocklisted_tags

    def error(self, message):  # pragma: no cover

        """ Amusingly, Python's documentation doesn't mention that this

        function needs to be implemented in subclasses of the parent class

        of parser.HTMLParser. This was found by fuzzing,

        triggering the following exception:

            NotImplementedError: subclasses of ParserBase must override error()

        """

        raise ValueError(message)

    def handle_starttag(self, tag: str, attrs: List[Tuple[str, Optional[str]]]):

        # Ignore the type, because mypy is too stupid to infer

        # that get_starttag_text() can't return None.