Extensible Markup Language (XML)
XML is a versatile markup language that provides a standard way to structure, store, and transport data. Developed by the W3C (World Wide Web Consortium), it is a simplified subset of SGML. While HTML is designed to display data, XML is designed to describe data, making it a crucial component of the Semantic Web.
An XML document consists of Elements, Attributes, and Entities. It must be well-formed, adhering to strict syntax rules such as case sensitivity and proper nesting. To validate the structure of an XML file, developers utilize a Document Type Definition (DTD) or XML Schema. For data retrieval, XPath is commonly employed, and for data transformation, XSLT is the industry standard.
Modern applications leverage XML for everything from Web Services and SOAP protocols to configuration files and Office Open XML formats. Detailed specifications can be found at the official W3C site and additional documentation is available through MDN Web Docs.