Document Validity
SGML (ENR+WWW) defines two kinds of validity. According to the revised Terms and Definitions of IS 8879 (from the public draft):
A conforming SGML document must be either a type-valid SGML document, a tag-valid SGML document, or both. Note: A user may wish to enforce additional constraints on a document, such as whether a document instance is integrally-stored or free of entity references.
A type-valid SGML document is defined by the standard as
An SGML document in which, for each document instance, there is an associated document type declaration (DTD) to whose DTD that instance conforms.
A tag-valid SGML document is defined by the standard as
An SGML document, all of whose document instances are fully tagged. There need not be a document type declaration associated with any of the instances. Note: If there is a document type declaration, the instance can be parsed with or without reference to it.
Read more about this topic: Standard Generalized Markup Language
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