How can I improve Emacs' font selection algorithm?

[Štěpán Němec]

I know I can use `set-fontset-font' to hardcode fonts used for specific
character ranges, but that's often inconvenient.

Emacs obviously sometimes does a rather poor job of font selection
(speaking about GUI Emacs under X in case it matters), e.g. when
selecting a fallback font for characters missing from the
default/specified font, it selects some ridiculously ugly (and very
differently-looking) font, even though there is a much better font
available for those characters; sometimes it even doesn't find a font at
all and displays just the encoding rectangles, even though there _is_
actually an appropriate font available (which I can e.g. set manually).
That's probably a bug.

Can something be done about that? I haven't been able to find anything
in the documentation. I imagine there could at least be something like a
`fallback-font-selection-order' listing the desirable fonts that could
be consulted during font selection as a crude workaround for the
problem.

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