Because Circle hasn't run a doc CI build at all.

There seems to be a separate bug; that the Cursive font-family isn't found by CI:
https://63196-1385122-gh.circle-artifacts.com/0/doc/build/html/gallery/userdemo/pgf_fonts.html

This is true for other PRs too (for example, the most recent #20949):
https://63199-1385122-gh.circle-artifacts.com/0/doc/build/html/gallery/userdemo/pgf_fonts.html

Though It did use to find the cursive font-family for the previous builds (on stable): https://matplotlib.org/stable/gallery/userdemo/pgf_fonts.html

/stable/ is built on my machine, not CI. You can't compare it to /devdocs/ or PR builds.

It appears that we use Comic (Neue, Sans MS) because it's already listed in font.cursive, but it's not available on CI. Felipa is also listed in that font list, and is explicitly installed on CI, so maybe we should switch to that instead?

As discussed during the dev call this is actually hiding a bug that I bisected back to #10339; the fix is clear looking at that PR, and should be something like

diff --git i/lib/matplotlib/backends/backend_pgf.py w/lib/matplotlib/backends/backend_pgf.py
index 749da55966..8706fed514 100644
--- i/lib/matplotlib/backends/backend_pgf.py
+++ w/lib/matplotlib/backends/backend_pgf.py
@@ -50,9 +50,14 @@ def get_fontspec():
         for family, command in zip(families, commands):
             # 1) Forward slashes also work on Windows, so don't mess with
             # backslashes.  2) The dirname needs to include a separator.
-            path = pathlib.Path(fm.findfont(family))
-            latex_fontspec.append(r"\%s{%s}[Path=\detokenize{%s}]" % (
-                command, path.name, path.parent.as_posix() + "/"))
+            try:
+                path = pathlib.Path(
+                    fm.findfont(family, fallback_to_default=False))
+            except ValueError:
+                pass
+            else:
+                latex_fontspec.append(r"\%s{%s}[Path=\detokenize{%s}]" % (
+                    command, path.name, path.parent.as_posix() + "/"))
 
     return "\n".join(latex_fontspec)

I'm moving to draft until the above is addressed. Thanks!

I think this just needs the change to the example to be reverted, but otherwise looks good.

Reverting the change does not seem to work.
https://63832-1385122-gh.circle-artifacts.com/0/doc/build/html/gallery/userdemo/pgf_fonts.html

I can remove the empty list from the font specification again, otherwise I'm not sure how to fix this error.

Actually there's another problem with the example, which is that it doesn't actulaly use the pgf backend (which is a pity...)
One needs to add e.g. backend="pgf" to each of the savefig calls.
That then shows another problem with the example, which is that family="cursive" doesn't work, which I guess could just be documented as a limitation of backend_pgf (because TeX has \rmfamily \sffamily \ttfamily but no \cursivefamily (or \calfamily, I guess).

I popped this back to draft, but feel free to put back on the queue

@FranzForstmayr Can you squash this into one commit so we do not have the commit and it's reversion in the history? You can do this with git rebase -i. Please ask if you need help!

Hmm, @FranzForstmayr did you intend to close this?

Yes, as there is a more recent PR (#22111) about this topic.