The CSS cue property is a shorthand property for setting both cue-before and cue-after property values. If two values are given, the first value is assigned to cue-before and the second is cue-after. Those two values must be separated by a space. If only one value is given, it applies to both properties. If loading of a URL fails, nothing should be played, but if the system does not have the ability to play it, the spec says the system should try to use some sort of cue (audio, visual or other).