When looking at the Dev Tools on F41 and Firefox, the site is serving up a copy of the Noto Sans web font as shown below:
KDE Font Management doesn’t render Noto Sans Regular as italicized. I also have the font packages installed, google-noto-sans-fonts.noarch 20240701-2.fc41
Looks like it’s also been reported on GH: Words become unnecessarily italic on Firefox · Issue #16760 · lichess-org/lila · GitHub