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Currently 'background' applies to the "WebVTT cue background box" while 'outline' applies to "list of WebVTT Node Objects". These might not be aligned, and it could look very odd indeed if the outline were floating outside the background. In related news, it might not be necessary to let the '::cue' pseudo-element (with no argument) match different boxes depending on the property. Other than outline, it doesn't seem like there are any properties where it would make any difference.
Also see https://github.com/w3c/webvtt/issues/235