SGML (Standard Generalized Markup Language)
SGML (Standard Generalized Markup Language) is a standard for how to specify a
document markup language or tag set. Such a specification is itself a document type
definition (DTD). SGML is not in itself a document language, but a description of how
to specify one. It is metadata.
SGML is based on the idea that documents have structural and other semantic
elements that can be described without reference to how such elements should be
displayed. The actual display of such a document may vary, depending on the output
medium and style preferences. Some advantages of documents based on SGML are:
1. They can be created by thinking in terms of document structure rather than
appearance characteristics (which may change over time).
2. They will be more portable because an SGML compiler can interpret any
document by reference to its document type definition (DTD).
3. Documents originally intended for the print medium can easily be re-adapted
for other media, such as the computer display screen.
The language that Web browser uses, Hypertext Markup Language (HTML), is an
example of an SGML-based language. There is a document type definition for HTML
(and reading the HTML specification is effectively reading an expanded version of the
document type definition). In today's distributed networking environment, many
documents are being described with the Extensible Markup Language (XML) which is
a data description language (and a document can be viewed as a collection of data)
that uses SGML principles.
SGML is based somewhat on earlier generalized markup languages developed at IBM,
including General Markup Language (GML) and ISIL.