I see that X for invalid UTF-8 stuff (if file mode is UTF-8 but the file
isn't really) and ? for characters that the terminal is not able to
display (e.g. when file mode is UTF-8 but terminal isn't) are just simply
displayed as a plain X or ?. This way it's hard to distinguish them from
other characters, e.g. a real question mark.
I'd like to request that these special X and ? characters be displayed in
inverse background/foreground or with extra brightness or some color so
that it's more trivial that there I don't see the contents of a file but
rather a special symbol denoting some unusal circumstance.