Thanks all for your interesting responses.
Unfortunately, my possibly poorly worded question remains unanswered. Let me
try again.
Consider the short example just used:
\documentclass{article}
\usepackage{fontspec}
\setmainfont{DejaVu Serif}
\begin{document}
^^^^fffd
\end{document}
When I run it, fontspec complains that it can't find the font. So obviously
"DejaVu Serif" is not installed, either on my system or anywhere in the bowels
of all the ~150,000 TeXLive (2019) files that have been installed in the TDS on
my machine.
So, is there a font name I can use in the \setmainfont{} command that is ALWAYS
available (upon TeX installation) when processing this LaTeX file with XeTeX?
Or always available after a certain version of a TeX installation?
I want to automatedly create such a LaTeX file that permits its user to declare
or override a default font for typesetting the Unicode Replacement character,
but which doesn't require the user to search for or declare such fonts at all
in the simplest case. One would think that every OpenType font supporting
Unicode glyphs would include a glyph for U+FFFD, but it doesn't appear to me
that that is the case.
All I want is for the computer-generated LaTeX file "to just work" out of the
box (so to speak), so that a naive user isn't faced with an error message the
first time they typeset it with something like TeXShop.
Doug McKenna
Mathemaesthetics, Inc.
