Prefix keywords are never removed

[frenic]

CSS properties are deprecated as they are marked as deprecated by the spec or are removed by vendor. But CSS keywords lives on forever at the moment (https://github.com/frenic/csstype/pull/36). Maybe this won't ever be a problem. But I'm a bit worried if old prefixed keywords results in too many combined keywords in the future. But lets see if that happens. This issue is for now just a friendly reminder to my self. 👴