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XSL is a language for expressing style sheets. An XSL style sheet is, like with CSS, a file that describes how to display an XML document of a given type. XSL shares the functionality and is compatible with CSS2 (although it uses a different syntax). It also adds: 1  A transformation language for XML documents: XSLT.
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.