Subtitles have (almost) nothing to do with the video codec. AVI, MKV, MP4, etc. are container formats that are used to multiplex multiple streams of media (video, audio, subtitles) into a single stream/file. In contrast, AV1, x264, HEVC, etc. are video codecs stored inside a multiplex. Perhaps confusingly, he MPEG LA (which standardised x264 and HEVC) is also responsible for the MP4 container.

> the MPEG LA (which standardised x264 and HEVC) is also responsible for the MP4 container.

The MPEG LA is just a licensing administrator formed by companies owning MPEG patents, they didn't standardize anything nor are they responsible for MP4 container.

The standards working group is MPEG, and according to Wikipedia the two are not affiliated ( https://en.wikipedia.org/wiki/MPEG_LA ).

You're absolutely right, sorry for getting this wrong. My mistake was to assume that the body that sells you the license is also responsible for the standardisation, but as you described, this is not true.

This is correct. The reason that the scene uses MKV is because it has better support for multiple audio, video, and subtitle streams and handles streaming as well as "streaming" from RAR compressed and split archive files.

x264 is an AVC encoding software by VideoLAN.

Damn. How embarrassing! Two major factual mistakes in two sentences while nitpicking at someone else's mistakes on the internet. I of course meant to write h264 ;-)

Unless you encode the subtitles directly on the video stream, this is a container solution, the previous Google codecs (vp8/vp9) uses webm, which in turn is based upon mkv, so I would assume AV1 supports subtitles as easily if it uses webm as well.

AV1 is a video format, it has absolutely nothing to do with subtitles. AV1 videos are usually inside MKVs anyway.

AV1 is most expected to be used inside webm, though mkv can also package it.

Furthermore webm/Matroska AV1 mapping isn't finalized, same situation with MP4.

webm is just a subset of MKV. They limited the codecs/options so that it would be easier have complete webm compatibility. (Whereas "full" MKV compatibility doesn't really make sense because it has ~infinite codecs you would need to support)