I believe XML has it baked directly into the official spec. JSONSchema is great, but I wouldn't call it a standard yet, and anecdotally I haven't seen it as oft

I believe XML has it baked directly into the official spec. JSONSchema is great, but I wouldn't call it a standard yet, and anecdotally I haven't seen it as often as I'd like in use (i.e. I either need to run kubectl --dry-run or use a separate third party solution to validate my changed yaml).