[xworld21]

The tagging work is still highly experimental. A major missing element is equation tagging: for now, you need to produce an 'associated MathML' file externally, for instance using LaTeXML. Even then, PDF readers do not support the MathML tags yet! If anything, I am sure that the LaTeX3 team would appreciate you posting minimal examples of mistagged PDFs.

If you want to produce accessible documents from LaTeX, you should convert to HTML. ATs such as screen readers just work miles better than with PDFs, and given the resources put in developing browsers compared to PDF readers, I don't think this will change any time soon. Luckily conversion from LaTeX to HTML is very feasible today, as proved by arXiv. (Shameless plug: I maintain BookML specifically to help lecturers with the LaTeX to HTML work)

[generationP]

A 50-page PDF loads a lot faster and shows a lot smoother than an HTML of equal textual length. And I've never seen any modern tools that turn TeX into multifile HTML (one per section).

[xworld21]

> A 50-page PDF loads a lot faster and shows a lot smoother than an HTML of equal textual length.

Very true! Although they are now comparable, if you rely on the browser native MathML instead of MathJax/LaTeX.

(You can test this on long arXiv HTML papers, e.g. https://ar5iv.labs.arxiv.org/html/1710.07304 is more than 60 pages as PDF. Mind you, the ar5iv default CSS is not great. I would use Latin Modern for formulas, at the very least.)

> I've never seen any modern tools that turn TeX into multifile HTML (one per section).

I believe all of them can do it out of the box now. I know for sure that LaTeXML, tex4ht and lwarp can split by chapter or section.