I wonder whether there are current facilities to write up a textbook / reference online. A well-known example is The Stacks Project. A similar case is Kerodon.
There are some basic machinery that is not common to other works:
Not that I know of. While working on the TeXmacs blog:
I became aware that a nice model could be that of creating all the (hyperlinked) content in TeXmacs as a series of .tm documents, which are then automatically exported to HTML (or also PDF). In this way the content remains browsable/editable within TeXmacs and also accessible via a web browser. Adding tags and also some cross-referencing / indexing should then not be difficult by extending the current facilities. For the moment cross referencing and the table of contents in the blog are written by hand. However the blog has already some scheme code which make a list of all the available documents, extract metadata and creates automatically a page with a consolidation of this list.
It is unclear what is the best, but certainly, it might be better if there is a modern online version of Bourbaki’s « Éléments de mathématique » to which everybody could refer to without ambiguity, so I guess that we can do something for TeXmacs to make this easier.