YAML supports tags, which could describe schema. JSON doesn't. Every method to describe schema for JSON will work for YAML, plus extras - like tags, and you could use node references to basically define types, and re-use them throughout.

Sometimes I feel like I'll go to Ruby hell for saying this, but as far as I can tell, XML is actually really good at that part... because it's a markup syntax, not a data syntax, while JSON and YAML are data syntaxes, not markups.