This kind of consideration has led me down the road towards XML, because it's the syntax that is powerful enough to model documents that bundle a mix of schemas

This kind of consideration has led me down the road towards XML, because it's the syntax that is powerful enough to model documents that bundle a mix of schemas(including source formats with their own idiosyncratic syntax). It is not a syntax anyone wants to type in or read, but that one advantage counts for a lot, given the right problem domain, because it pushes the semantic content to the center.