There are many standards or recommendation related to SGML, which define SGML DTDs, ipmlement style sheets, or linking.
SGML DTDs:
Style languages:
DSSSL, like SGML, is declarative in nature, and has no biases toward national language, writing direction, or character set. All style specifications in DSSSL are made by describing final formatting results, not by describing the algorithms to be used create the formatting result. In other words, DSSSL enables us to specify how data appears in output (print or electronic) but not how to create that format.
a style sheet language that allows authors and users to attach style (e.g., fonts, spacing, and aural cues) to structured documents (e.g., HTML documents and XML applications). By separating the presentation style of documents from the content of documents, CSS2 simplifies Web authoring and site maintenance.
Linking languages:
allows the description of virtually any kind of connected or temporal information in SGML. It covers many different aspects on linking, as well as a full repertoir of features for multimedia purposes, including virtual time, scheduling, synchronisation and so on. Is incredibly complicated.
"Linking, Segmentation, and Alignment." Chapter 14 (pages 393-454) of the TEI Guidelines for Electronic Text Encoding and Interchange.