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http://lists.w3.org/Archives/Public/public-texttracks/2014May/0062.html WebVTT is currently not capable of providing acceptable padding on every line of a cue, even it were possible to set the padding on it, because of this effect: http://software.hixie.ch/utilities/js/live-dom-viewer/saved/3014 box-decoration-break could be part of the solution: http://dev.w3.org/csswg/css-break/#break-decoration
v1 since creating captions that don't look terrible isn't possibly without this.