Comment 9 for bug 932958

Now I think about it more, even just ensuring that monospace fonts fall back to other monospace fonts wouldn't solve the problem, for two reasons. First, monospace fonts can be different widths, so the fallback might be wider than the original. And second, the character might not be present in any monospace fonts at all, but still be present in a proportional font, and it would be better to fall back to a proportional font than to nothing at all.

So ideal behavior, perhaps, would be ensuring that a fallback character for a monospace font is the same width as the original font -- by centering it if it is narrower, and by scaling it down if it is wider.