Text Encoding Inititative (TEI or TEI-XML)
TEI Guidelines for Electronic Text Encoding and Interchange: https://www.tei-c.org/release/doc/tei-p5-doc/en/html/index.html
The Journal of the Text Encoding Initiative (JTEI): https://journals.openedition.org/jtei/
DocBook
DocBook5: The Definitive Guide: https://docbook.org/
Balisage customization of DocBook V5: https://www.balisage.net/tagset.html
Music Encoding Initiative (MEI)
About: https://music-encoding.org/
MEI Guidelines: https://music-encoding.org/guidelines/v4/content/
JATS: Journal Publishing Tagset
Home: https://jats.nlm.nih.gov/index.html
Tagset: https://jats.nlm.nih.gov/publishing/
Comic Book Markup Language (CBML)
About: http://dcl.ils.indiana.edu/cbml/
Article about CBML: http://www.digitalhumanities.org/dhq/vol/6/1/000117/000117.html