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XSL is used to refer to a family of languages used to transform and render XML documents. CSS is a style sheet language used for describing the presentation semantics (the look and formatting) of a document written in a markup language. Hence XSL and CSS are not the same and not interchangeable.
JSON is another format of expressing data, just like XML. But, JSON is very simpler than XML, and tiny than XML. So, it is becoming popular in the web world to choose the JSON notation over XML since JSON notation are usually shorter, and less data to be transmitted if at all they were to be.