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http://dev.w3.org/html5/webvtt/#dfn-webvtt-text-position-cue-setting "One of the following strings: "start", "middle", "end"" Why are left and right not allowed? In https://www.w3.org/Bugs/Public/show_bug.cgi?id=23404 it looks like it was intended to be supported.
Looks like the syntax was not updated together with the rest.