It doesn't actually exist, shouldn't be registered.
commit 8ecd4d643abc915d70db6fab46b17d32dfe64360 Author: Bastien Nocera <hadess@hadess.net> Date: Wed Apr 2 11:38:01 2014 +0200 Remove *.CSSL glob for CSS files It was probably meant to be *.CSS1 (one, not "L") which got badly transposed when the KDE and GNOME databases were merged more than 10 years ago. As nobody seems to have missed *.CSS1 and *.CSSL is wrong, remove it. https://bugs.freedesktop.org/show_bug.cgi?id=76940
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