> "YAML" should mean something.

Sure, agreed. Another comment notes that GitHub probably should call this their own proprietary subset of YAML, and I wouldn't object to that.

> Well, hopefully they also prioritize fixing that?

I expect they won't, since it's not clear what version of YAML they even aim to be compatible with.

However, I don't understand why engineers who wouldn't jump off of a bridge because someone told them to would follow a spec to the dot just because it exists. Specifications are sometimes complicated and bad, and implementing a subset is sometimes the right thing to do!

GitHub Actions, for example, doesn't make use of the fact that YAML is actually a multi-document format, and most YAML libraries don't gracefully handle multiple documents in a single YAML stream. Should GitHub Actions support this? It's entirely unclear to me that there would be any value in them doing so; subsets are frequently the right engineering choice.