i think one of the major reasons JSON took this role from XML is it has a tighter syntax and easier to understand rules, so serialization, hydration and deserialization became easier to optimize over time. I know over the wire JSON packs more tightly than XML too and that used to matter a lot more than I might now depending on the end users of your app too. I think these validation specifications though are all trying to solve the same problem: describing your API to users without human interaction. It turns out to be a tough problem