Chemical formula HTML

        Args:
            reversed (bool): reversed text for leftmost atom groups

def formula_html(self, reversed_=False):
        """Chemical formula HTML

        Args:
            reversed (bool): reversed text for leftmost atom groups
        """
        if self.H_count == 1:
            text = "H"
        elif self.H_count > 1:
            text = "H<sub>{}</sub>".format(self.H_count)
        else:
            text = ""
        seq = [self.symbol, text, self.charge_sign_html()]
        if reversed_:
            seq = reversed(seq)
        return "".join(seq)