They might be rarely considering code formatting specifically, but the claim that one language is a superset of another really does imply that all valid instances of the latter are also valid instances of the former (and are similarly parsed).

The old spec of YAML 1.2 section 1.3 explicitly said:

YAML can therefore be viewed as a natural superset of JSON, offering improved human readability and a more complete information model. This is also the case in practice; every JSON file is also a valid YAML file.

The revised spec 1.2 revision 1.2.2 (2021-10-01) no longer contains that sentence; but still says, in section 1.2:

The YAML 1.2 specification was published in 2009. Its primary focus was making YAML a strict superset of JSON.

and in section 6.8.1:

Note that version 1.2 is mostly a superset of version 1.1, defined for the purpose of ensuring JSON compatibility.

Given all these claims, Patrick Stevens’ observations that YAML really isn’t a superset of JSON, because YAML can’t handle all JSON number literals, and tabs as whitespace, really is surprising. At least to me.

When previously JavaScript/ECMAScript 2018 was found not to be a JSON superset, at least it was about unescaped occurrences of little-used characters U+2028 LINE SEPARATOR and U+2029 PARAGRAPH SEPARATOR in string literals. And even that got fixed (by allowing the unescaped characters) in ECMAScript 2019.

[YAML 1.2]: https://yaml.org/spec/1.2-old/spec.html#id2759572 [YAML 1.2 revision 1.2.2]: https://yaml.org/spec/1.2.2/ [ECMAScript 2019 feature Subume JSON]: https://v8.dev/features/subsume-json