sTeX3 is a complete rewrite of sTeX - An infrastructure for semantic preloading of LaTeX documents - in the LaTeX3 framework. It has just been released to CTAN the Comprehensive TeX Archive Network, and will be part of TeXLive 2022 due out in April.
After almost 15 years, sTeX needed a complete makeover to regularize syntax, weed out unused and deprecated features, and synchronize it better with the MMT system and functionality.
Compared to sTeX1, the new implementation does away with the need for external systems in the PDF workflow, allows to add sTeX functionality to arbitrary document classes, and completely reworks the XML-based content generation. Instead of using a LaTeXML fork we use RusTeX a new TeX engine that directly interprets the sTeX3 packages and generates semantically annotated XHTML5 which can then be harvested by MMT for further knowledge management services.
Further development of the sTeX ecosystem (the packages, an IDE, RusTeX, …) will continue on sLaTeX group on GitHub.