JSON's syntax, at least, is defined. Also using JSON implies that extra stuff will be ignored and missing stuff rendered as null rather than causing the parser

JSON's syntax, at least, is defined. Also using JSON implies that extra stuff will be ignored and missing stuff rendered as null rather than causing the parser to crash - it's a forgiving format. Meanwhile it's a lot lighter than XML/HTML. And of course, I'm envisaging use of the "Accept" header to pick a format. Browsers ask for HTML. API users might ask for JSON.