There is a problem with .vtt subtitles in IE11. Problem exists for 4.12.x branch. In Google Chrome, Opera and Firefox works fine. Reverting to 4.10.x or 4.11.x branch makes subtitles working perfect in IE.
This sounds like a problem with video.js, and not the video.js website. Take a quick search for any issues in the video.js repo and if nothing comes up I'd recommend opening a new issue there.
There is a problem with .vtt subtitles in IE11. Problem exists for 4.12.x branch. In Google Chrome, Opera and Firefox works fine. Reverting to 4.10.x or 4.11.x branch makes subtitles working perfect in IE.