In the past, not all browsers provided a UI for turning subtitles on
and off so turning them on by default was the best solution. Now, all
major browsers implement a proper UI for subtitles, so the default
should be subtitles off which is what most users would expect.
Current browsers support WebVTT subtitles and also placing srt
subtitles there just makes the selection menu too long. In the past,
it was a good idea to include both because it was not clear yet which
system browsers would support, but it should not be necessary anymore.