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In section 3.5.1 Applying CSS properties to WebVTT Node Object: http://dev.w3.org/html5/webvtt/#applying-css-properties-to-webvtt-node-objects "The 'text-align' property on the (root) List of WebVTT Node Objects must be set to the value in the second cell of the row of the table below whose first cell is the value of the corresponding cue's text track cue alignment: Text track cue alignment 'text-align' value Start alignment 'start' Middle alignment 'center' End alignment 'end' " The second column should contain valid CSS values for the 'text-align' property (I assume 'left', 'center' and 'right').
'start' and 'end' are valid values, and I think they are deliberate. http://dev.w3.org/csswg/css3-text/#text-align0
Indeed, I was reading the old spec. Thank you Simon for pointing that out.