I am reading about IMSC and the docs say it should be in XML https://developer.mozilla.org/en-US/docs/Related/IMSC
Meanwhile, I have an RTMP stream with embedded caption data from the HLS manifest. When I look at the fragments, it all looks like binary to me rather than XML. I actually checked all network traffic from the browser and only see the manifest and fragment calls. In sample players online I DO see the captions getting built up and displayed, but I'm not sure how they go from Manifest -> XML.
As far as I can tell devs should be using https://developer.mozilla.org/en-US/docs/Related/IMSC/Using_the_imscJS_polyfill if they want to show live captions.
ISMC is carried inside fragmented mp4 files, they are not stand alone text files like WebVTT.