One of the screenshots was of the Arabic version of What is Unicode; went over to the Unicode website to have a look around, and came upon the Last Resort Font:
Last Resort Font
The Last Resort font is a collection of glyphs to represent types of Unicode characters. These glyphs are designed to allow users to recognize that an encoded value is one of the following:
- a specific type of Unicode character
- in the Private Use Area (no private agreement exists)
- unassigned (reserved for future assignment)
- one of the illegal character codes.
Which sounds like the sort of thing we need for daily life. In its absence things go horribly wrong and no one knows why, because unrepresentable characters have no appropriate "missing" glyph.