As the man said in the original question, its all about accessibility (for people needing screenreaders and such to use ebooks).
You can read something about it here:
https://inclusivepublishing.org/tool...ty-guidelines/
where there are also links to the specs. (WCAG 2.0, or a subset is among the criteria.)
You might also want to consider KevinH's brilliant plugin for Sigil (Access-Aide) and Doitsus ACE-validation plugin.
Regards,
Kim