It would be nice if U+FFFD was rendered as the replacement character glyph, rather than resulting in a warning and being replaced with a normal '?' character. This way there would be a standard way to represent unrepresentable characters before feeding data to Prince.
Unicode U+FFFD support
Currently we treat this character like any other character, so it will be displayed if there is a glyph for it and replaced with ? if not. However, many older fonts, including Times New Roman and Arial, do not include glyphs for this character. If you are using the DejaVu fonts or Arial Unicode MS then it should appear. Probably we should add Arial Unicode MS as a default fallback font, on Windows at least.
Actually it seems we are already using Arial Unicode MS as a last resort fallback font on Windows, so this problem should only occur on other platforms, like Linux and MacOS X.