The user has made no preference known to the system
The user has notified the system that they prefer an interface that removes or replaces the types of motion-based animation that either trigger discomfort for those with vestibular motion sensitivity, or distraction for those with attention deficits
Values for the Gtk.Settings.gtk_interface_reduced_motion and Gtk.CssProvider.prefers_reduced_motion properties that indicates the preferred level of motion animations.
This information can be used inside CSS via media queries.
Since
4.22