Json is not popular because it saves bits but because it's easily readable and maps 1:1 with js objects. I've worked with xml and xslt and while I see the advantages I still prefer json for anything. Your validation should come from annotated objects on the backend anyways which should be a single source of truth and then it doesn't matter which markup format you are validating
I can kind of see what you're saying, but what does "annotated objects on the backend" mean. If there is not a standard I can only hack a solution together. I cannot ensure anything with 100% certainty, instead I'm left guessing if my logic is correct.
With a standard, however, I can be sure that what I intend is being validated and if my logic is wrong then I can change it to match the spec.
Json schema maybe http://json-schema.org/
Its regrettable that such a vast ecosystem exists for a markup language to facilitate its use in data transfer, transformation and validation.
It seems like a waste that so many working standards seem to have just been tossed away with nothing to put into their place because Json saves a few bits on the wire.
Also regrettable is the fact that there seems to be resistance in developing similar standards for json.