[org-cite, oc-csl] citation fontification and previews?

[Bruce D'Arcus]

In an earlier discussion, people raised the issue of fontification and previews.

But I'm wondering what Nicolas has already implemented, and what he or
others might still add.

Details:

John Kitchin (based on his experience with org-ref):

https://lists.gnu.org/archive/html/emacs-orgmode/2021-04/msg00438.html

He suggested a green face for known references, and red for unknown.

I think Nicolas implemented this basic idea somewhere, but I am not
noticing it; I don't see any difference with incorrect keys.

John also suggested a tooltip preview of the roughly formatted reference.

Again, I thought Nicolas had earlier implemented this, but I don't see
it ATM; hovering over keys doesn't do anything.

András Simyoni:

https://lists.gnu.org/archive/html/emacs-orgmode/2021-04/msg00289.html

He was thinking about using citeproc-el to attach some kind of preview as well.

"I'm thinking about implementing a "fontification" solution which would
use citeproc-el with a standard style to produce nice preview-like
representations of the citations in the buffer. This would require
basically the same pieces of information as citation export I think,
although it might be made strictly  local, working only with the
single citation object plus the bibliography information."

Timothy:

https://lists.gnu.org/archive/html/emacs-orgmode/2021-04/msg00456.html

He was thinking about previews as well, with overlays:

"I think what would be ideal, would be if common citation styles could
define a method which produces a display string, like "Goaziou et al.
(2021)". If nothing is defined, then no overlay should be produced."

And my reply to him, based on the example of Zettlr:

https://lists.gnu.org/archive/html/emacs-orgmode/2021-04/msg00462.html

I think that example would imply overlays as well, though I don't
really understand all the technical details or org-specific pros and
cons (tooltips vs overlays and such), but am just wondering:

1. what is already included, and what should ship default? Perhaps
Andras' idea would fit for oc-csl, since it already depends on
citeproc-el?
2. if some is not included, say preview, how would one add that? I
assume the API supports it; so one would just install, say, a little
package and add that as a "follow" processor? And one could add
multiple such processors?

Bruce