Validating sgml parser
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.
There need not be a document type declaration associated with any of the instances.
Tags can be replaced with delimiter strings, for a terser markup, via the SHORTREF feature.
This markup style is now associated with wiki markup, e.g.
Reference-free reflects the HTML requirement that entity references are for special characters and do not contain markup.
SGML validity commentary, especially commentary that was made before 1997 or that is unaware of SGML (ENR WWW), covers type-validity only.
HTML was theoretically an example of an SGML-based language until HTML 5, which browsers cannot parse as SGML for compatibility reasons.