XSLT
"Extensible Stylesheet Language Transformations" — W3C standard 1999 for transforming XML into other XML, HTML or text. A functional language (Turing-complete).
Pattern-based <xsl:template match=...> that match via XPath and produce output. XSLT 1.0 is ubiquitous (browser support, though Chromium is pulling it back in 2025). XSLT 2.0/3.0 (Saxon) adds a lot — functions, schema awareness, streaming.
Lives on in publishing (DocBook → PDF), financial reporting (XBRL), legacy enterprise. Completely absent from modern web apps.