@kekulta Thank you, this is incredible work. If I'm understanding the change correctly, the POT is cached in session storage and can be decoded. I think this approach will work great.

I only have a few things to add before you clean things up:

• As you already commented in your change, some parts of the POT implementation like the session storage keys might be unstable. If possible it would be good to keep your integration as resistant to change as possible.
• If for some reason the POT extraction fails, it would good if we could still fallback to the existing InnerTube solution.

Some other minor comments which I think you're already aware of:

• base64 functions can be reused, instead of rewriting them in a few places.
• The SRT conversion can be done using the SrtParser already being used with the InnerTube transcription.
• There's no need to try to support legacy browsers - asbplayer will never run in a browser that isn't modern-ish.

One other thing - the original idea to support YT's machine-translated tracks will need the user to specify their target language somewhere. Naturally, I think this would be another setting. But let me know if you had other ideas.

Thanks again for this contribution. I used your PR as a starting point to integrate POT decoding as a fallback method for fetching YT subs. In particular, the POT seems to provide the only reliable way to fetch subs for members-only videos.