public function headerIdCallback(string $raw): string
    {
        $id = mb_strtolower($raw);
        $id = preg_replace('/[^\w]+/u', '-', $id);
        $id = 'j2h.'.$this->c::mbTrim($id, '', '-');

        $this->header_id_counter[$id] = $this->header_id_counter[$id] ?? 0;
        ++$this->header_id_counter[$id]; // Increment counter.

        if ($this->header_id_counter[$id] > 1) {
            $id .= '-'.$this->header_id_counter[$id];
        }
        return $id; // `id=""` attribute value.
    }