Currently we assume these are "start", "center", "middle" or "end", which was correct in a previous draft of the spec: https://www.w3.org/TR/2014/WD-webvtt1-20141111/#dfn-webvtt-text-position-cue-setting The released spec uses "line-left", "center" and "line-right": https://www.w3.org/TR/webvtt1/#webvtt-position-cue-setting PiperOrigin-RevId: 307371066
