[aepiepaey]

> Of course you can make an “.h264” with opus audio but in reality no one does that.

People definitely do that. And even if they didn't, significantly more people create h264+flac releases, and flac is also supported by much fewer devices than e.g. AC3.

E.g. .mkv does convey useful information. A player needs to support the container format as well, not just the codecs for the streams it contains. Different containers also support different features (not all containers are equal), and there are tools that only work for some containers.

The signal provided is only accidental and unreliable as a proxy for audio or video codec, not in general.

.webm is also a bit of a bad example here, since it's just a subset of matroska (mkv), but more importantly it initially only supported a single video codec and a single audio codec (VP8 & Vorbis). Of course, that has changed since, with the addition of support for VP9, AV1, and Opus.