TextTrackExtensions uses non-standard addRule
Created by: yurydelendik
The code at setCueStyle [1] uses non-standard addRule function. The spec [2] does not list this method, so not all browsers shall implement it.
[1] https://github.com/Dash-Industry-Forum/dash.js/blob/development/src/streaming/extensions/TextTrackExtensions.js#L396 [2] http://www.w3.org/TR/DOM-Level-2-Style/css.html#CSS-fundamental