Some alphabet soup

SGML

Standard Generalized Markup Language

HTML

Hypertext Markup Language

W3C

World Wide Web Consortium

XML

eXtensible Markup Language

DTD

Document Type Definition (or Declaration)

CSS

Cascading Style Sheet

XSLT

eXtensible Stylesheet Language - Transformations

RelaxNG

Not an acronym, as far as we know

Oh, and then there's also

TEI

Text Encoding Initiative