Add VTTCue and WebVTT support
Supported by all other browser vendors in Chrome, FF, Safari.
Also implement ::CUE pseudo-element to enable support in CVAA.
Currently all player developers need to create their own subtitles/captions implementations due to browser having fragmented support in all of the above which makes supporting accessibility very hard and cumbersome.
Having you support this will ensure better interoperability.
Please consider migrating the solution to IE11 as well as you still have it on platforms which are widely supported and EOL is not near as it seems.
Also opened as an issue in the platform tracker: