Any application which substitutes missing glyphs for PUA characters,
when a valid font which covers those code points is active,
should be considered non conformant.
From Unicode 4.0, Chapter 3, Conformance, rule C8:
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_C8 A process shall not assume that it is required to interpret any particular coded character representation._
o Process that interpret only a subset of Unicode characters are allowed; there is no blanket requirement to interpret _all_ Unicode characters.
o Any means for specifying a subset of characters that a process can interpret is outside the scope of this standard.
>>
I take this to mean that any application can refuse to interpret PUA code points and still be conformant.
I do not find any rules as to what an application ought to do with code points that it does not interpret. Unless I'm missing something, substitution of a missing glyph indication would be conformant.
I think it would be better if such an application indicated this in some other way than by the same missing glyph that it would use to indicate a character was not found in the current font, but I don't see that Unicode imposes any such requirement.
Jim Allan
