The mdoc language supports authoring of manual pages for the man(1) utility by allowing semantic annotations of words, phrases, page sections and complete manual pages.
From GNU groff's mdoc(7):
The -mdoc package is a set of content-based and domain-based macros used to format the BSD man pages.
I'm not 100% sure when exactly mdoc was introduced, but the earliest appearance I can find is in /share/tmac/tmac.doc in 4.3BSD-Reno [1], whose file header gives it as version 5.9, dated 24 July 1990 (a modified version of this same file appears in the earliest release of GNU groff I could find, version 1.02 from 2 June 1991 [2]). And taking a look at the mdoc language reveals that it is indeed very much a semantic markup, even more so than most markups that otherwise receive the moniker.
And even the unmaintained (and relatively featureless) version of man(1) that ships with my Slackware 14.2 installation supports it by default, via the magic of groff -mandoc:
mandoc — Use this file in case you don't know whether the man macros or the mdoc package should be used. Multiple man pages (in either format) can be handled.
(that's from groff_tmac(5); meanwhile man.conf(5) says that -mandoc is used by default).
A look at the source (in /usr/share/groff/1.22.3/tmac/andoc.tmac on my system) reveals some troff macro trickery to load either the man package or the mdoc package based on which macros the manpage is trying to use, and then reload the manpage with the appropriate package.
4.3BSD-Reno has an equivalent, too (in /share/tmac/tmac.andoc), whose header gives it as version 5.4, dated 28 July 1990—it strikes me as considerably simpler, but the basic idea is the same. Ye olde groff's was similarly simple, so I'm sure there are good reasons for the complexity that's since been introduced.
The point of all this being: man(1) has supported semantic markup by default since before Linux or any of the modern BSDs existed. On BSD it's been the preferred markup for manpages for ages, afaict, but it's never really caught on in the Linux world, despite having been supported—perhaps because it's always been more popular to write manpages in something like POD or LinuxDoc or DocBook or AsciiDoc or Markdown or some other markup and programmatically convert them to roff(7) input, and those formats don't offer the semantic richness or detail for the domain of manpages that mdoc(7) does, so converting to man(7) makes more sense in that case.
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[1]: https://minnie.tuhs.org/cgi-bin/utree.pl?file=4.3BSD-Reno/sh...
[2]: http://git.savannah.gnu.org/cgit/groff.git/commit/macros?id=...
