Hello.
I think I found a typo in the hier(7) manpage.
This part specifically:
/usr/share/sgml/mathtml
MathML DTD (optional).
I believe that it should be "mathml" instead of "mathtml".
This typo is present in the latest release of this manpage, as it can be found in the one displayed online (https://man7.org/linux/man-pages/man7/hier.7.html).
FHS 2.3 (https://www.pathname.com/fhs/pub/fhs-2.3.pdf) page 27 states:
"The following directories, or symbolic links to directories, must be in /usr/share/sgml [...] mathml"
This shows that "mathml" is the intended spelling.
Regards,
Grigoriy.
