XML has a relatively small specification. For some time I used to "print" web pages into PDF (or XPS) and I remember that XML 1.0 specification was three times shorter than that of YAML (it was YAML 2, I think, I don't quite remember). And XML included a) serialization itself, b) simple grammar specification in the form of DTD, c) things like internal references from one element to another, d) basic support for other notations, so that you could add, say, LaTeX math notation and formally define that this element's content is in this notation. I do not think (b), (c) or (d) were part of YAML or any other similar format.