fi and fl are common typographic ligatures. They are the first ligatures
shown in the Wikipedia article:
https://en.wikipedia.org/wiki/Typographic_ligature
https://en.wikipedia.org/wiki/Typographic_ligature#/media/File:Ligature_drawing.svg
The Zertifikat becomes Zertifikat, and kostenpflichtig becomes
kostenpflichtig.
It depends on the fonts used by the web site and the available fonts on
the Linux system, whether they can be displayed and printed correctly.
It could also depends on other settings, e.g. composite characters could
automatically be "decomposed".
If in doubt, let the web site use its own fonts, e.g. do not disable the
setting "Allow pages to choose their own fonts, instead of your
selections above".
Or try the "Noto" fonts - they are meant to prevent any missing characters.
To learn more, install the package "unicode" and try:
$ unicode "ligature fi"
$ unicode "ligature fl"
Regards,
Hartmut Buhrmester
