enhancement: option to disable font fallback

[100amps]

In order for me to spot any missing codepoints in my font it would be helpful if I could disable the CSS font fallback (with a checkbox perhaps), which is replacing my missing glyphs with those from a default fallback font in the blocks of proofing text. (I'm assuming that feature is what's causing my grief). I would rather see the "missing glyphs". A more helpful behavior would be to allow my font's .notdef or replacement glyph to show in these spots instead. Thanks for listening.