[Daiz]

Unfortunately, as the link describes, Netflix only makes this available for a very limited set of languages, while everyone else is stuck with the extremely limited text-based standards.

Frankly, those text-based subtitle standards are quite maddening on their own. Netflix's text-based subtitle rendering seems to support a much wider set of TTML features than what it actually allows subtitle providers to use - so if these restrictions were to be slightly relaxed, providers could start offering better subtitles for anime immediately with no additional effort from Netflix.

[mook]

What Netflix supports on their main website might not be what they care about, though; you used to be able to watch Netflix on the Nintendo Wii, and they probably still have some users on stupidly old smart TVs.

[Mindwipe]

Also fun fact - subtitles did not work on the Wii at all if you were running a video streaming service!

The BBC spent literally years trying to engineering something that did not result in it being unable to playback video smoothly and failed.

[crtasm]

Fast forward to 2025 and BBC's streaming app on ApppleTV only just added subtitles; vastly more powerful hardware but so many restrictions from Apple on how developers use it.