XSLT is a functional language for manipulating primarily tree-structured data and information, such as you might find in XML or JSON or HTML for example.
Someone recently asked me for an XSLT course that includes XSLT 4; usually i teach XSLT 3, the latest, but 4 is in preparation. It's not quite ready for production use, but if you want to be ahead of the curve the course might go ahead in the next month or two.
DM if interested.