This has become a standard on analog clocks and watches (presumably to avoid confusion with VI), but for some reason IX and XI (for 9 and 11 respectively) is fine.
Personally I’d like to see IX and IIIIIIIIIII.
ETA: I guess IX and XI are ‘fine’ because they’re not upside down, but my point still stands.
This reminds me of a podcast (an episode of This American Life maybe) where one of the reporters talks about being surprised when someone mentions Nazi’s in The Sound of Music because they only ever saw up to the intermission and thought that was the entire movie.