The Standard Generalized Markup Language (SGML; ISO 8879:1986) is for defining generalized markup languages for documents. ISO 8879 Annex A.1 defines generalized markup:

Generalized markup is based on two postulates:

• Markup should be declarative: it should describe a document's structure and other attributes, rather than specify the processing to be performed on it. Declarative markup is less likely to conflict with unforeseen future processing needs and techniques.
• Markup should be rigorous so that the techniques available for processing rigorously-defined objects like programs and databases can be used for processing documents as well.