Which subtitle and CC formats are supported on native Safari
The Safari browser on iOS uses the native Safari video player.
Safari on the desktop also uses its native player when using FairPlay or when the useNativePlayback flag is enabled.
WebVTT
For displaying webvtt subtitles, Safari on iOS requires the SUBTITLES attribute in the EXT-X-STREAM-INF tag of the m3u8 file.
This SUBTITLES attribute is optional on browsers such as Chrome and Firefox and when Safari does not use its native video player.
TTML
TTML subtitles aren't supported through native Safari. However, THEOplayer should be able to handle TTML subtitles on desktop Safari as long as FairPlay is not being used and the useNativePlayback flag is not enabled.
CEA608 and CEA708
CEA608 and CEA708 subtitles are supported.
Known Limitation
When multiple subtitles with mode hidden are configured, only one will fire its events such as addcue and cuechange when using Safari's native player.
Resources
The following resources provide more information: