Do you consider JSON as a prevailing standard for data-interchange?
Do you consider it is worth to consider JSON instead of XML?
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JSON is presumably very efficient for data-exchange, but has limited application-value for elaborate data-description. While JSON can considerably reduce the overhead (~30%) which is important if data has to be transferred, e.g. in applications using websockets or AJAX environments, there are fields where more complicated descriptive capabilites are necessary, like e.g. for MathML which is based on XML. So from that point of view, and given the wide range of applications for XML, i think we are talking about two slightly different things, where none could really replace the other completely.