SGML

An international standard (ISO 8879:1986) metalanguage for defining markup languages that describe the structure and content of electronic documents. SGML introduced foundational concepts including descriptive markup (tagging what content is, not how it should look), document type definitions (DTDs) that formally specify document structure, and the separation of content from presentation. SGML is the direct ancestor of both HTML and XML. Its emphasis on structural markup rather than visual formatting proved essential for document accessibility, as structured documents can be automatically transformed into braille, large print, synthetic speech, and other accessible formats.