How can tagged PDFs be created that support Universal Accessibility and reflowing?

Ross Moore talked about tagged pdf at TUG 2010:
http://river-valley.zeeba.tv/tex-mathml-for-tagged-pdf-the-next-frontier-in-mathematical-typesetting/

He mentioned that he uses an "extension" of pdfTeX, but I don't know the details. There would certainly need to be a lot of work done to transform the structure of a LaTeX document into a form that a re-flowable PDF viewer could take advantage of.

The short answer as of 2020 is that tagged PDF is not possible with LaTeX.

There are several approaches under more or less active development to resolve this:

• Ross is working on his package/script but it is unofficial.

• Andy has released package accessibility but the created tags are flawed in a way you just get a blank screen in reflow.

• Ulrike has released package tagpdf but it's experimental and not meant for document production.

You may get a different answer for plain pdfTeX (note: without La) but for a majority this will be too complicated. Hopefully we can modify this answer in a few years.

I also read, that in the future, the LaTeX Core Team plans to provide functionality required for "tagged PDFs", first as an Add-on and later integrated into the kernel itself. Before this is implemented, I understood that it is nearly impossible to provide a working solution without restrictions, breakage or manual tagging/adjustments as also stated in the answer above.

Source: Quo vadis LaTeX(3) Team — A look back and at the upcoming years