docs: def _get_shortcut_prefix(self, user_agent, standart_prefix):
          """
          Returns the shortcut prefix of browser.
  
          :param user_agent: The user agent of browser.
          :type user_agent: str
          :param standart_prefix: The default prefix.
          :type standart_prefix: str
          :return: The shortcut prefix of browser.
          :rtype: str
          """
          # pylint: disable=no-self-use
  
          if user_agent is not None:
              user_agent = user_agent.lower()
              opera = 'opera' in user_agent
              mac = 'mac' in user_agent
              konqueror = 'konqueror' in user_agent
              spoofer = 'spoofer' in user_agent
              safari = 'applewebkit' in user_agent
              windows = 'windows' in user_agent
              chrome = 'chrome' in user_agent
              firefox = (
                  ('firefox' in user_agent)
                  or ('minefield' in user_agent)
              )
              internet_explorer = (
                  ('msie' in user_agent)
                  or ('trident' in user_agent)
              )
  
              if opera:
                  return 'SHIFT + ESC'
              elif chrome and mac and (not spoofer):
                  return 'CTRL + OPTION'
              elif safari and (not windows) and (not spoofer):
                  return 'CTRL + ALT'
              elif (not windows) and (safari or mac or konqueror):
                  return 'CTRL'
              elif firefox:
                  return 'ALT + SHIFT'
              elif chrome or internet_explorer:
                  return 'ALT'
              return standart_prefix
          return standart_prefix