XML
Extensible Markup Language — a W3C-standardized markup format with named tags, attributes, and namespaces, designed in the late 1990s for self-describing, human-readable structured data interchange.
XML (Extensible Markup Language) is a W3C recommendation finalized in 1998 as a simplified subset of SGML, intended as a portable text format for structured data. Its core design choices — paired opening/closing tags, attributes on elements, explicit namespaces via URI prefixes, and mixed content (text interleaved with child elements) — were aimed at document publishing as much as data interchange. Companion specifications layered substantial power on top: XML Schema (XSD) and DTDs for validation, XPath and XQuery for querying, XSLT for transformation, and XML Signature/Encryption for security. In the early 2000s XML was the default wire format for SOAP web services and feeds like RSS and Atom. After 2010 it lost the public API battle to JSON, but it remains dominant in domains that need mixed content, deep transformation pipelines, or cryptographic signing of arbitrary tree fragments — notably SAML, XBRL, DocBook, JATS, and DITA. XML's verbosity is real, but its data model is strictly richer than JSON's: it can natively express ordered mixed text-and-element content that JSON has to encode awkwardly.