prefers-color-scheme CSS media feature allows web designers to reflect the user’s operating system appearance option to a website to offer a dark mode in particular. The available values are
no-preference. The third value, however, has been removed with Firefox 79 because it was dropped from the CSS spec.
According to the intent post,
no-preference was relatively recently introduced, and no browser has enabled the support. It means the risk of the removal should be very low.