allow "center".
This value (and not "middle") is listed in
https://w3c.github.io/webvtt/ ( WebVTT: The Web Video Text Tracks Format, Draft
Community Group Report, 21 December 2015).
Leaving the behavior for "middle" unchanged.
It was the value listed in older drafts, e.g.
https://www.w3.org/TR/2014/WD-webvtt1-20141113/ ( W3C First Public Working
Draft 13 November 2014 )
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