Wikipedia
mandoc (historically called mdocml) is an ISC licensed utility for formatting man pages, specifically those written in the mdoc macro language. Unlike the groff and older troff and nroff tools predominantly used for this purpose, mandoc focuses specifically on manuals and is not suitable for general-purpose type-setting.
Development began in November 2008 specifically to produce CSS-enabled HTML forms of manuals in response to the limitations of groff. mandoc gained initial text-mode output in February 2009. It was then showcased at AsiaBSDCon-2009. In summer of 2010, mandoc was the subject of a NetBSD-mentored Google Summer of Code project for producing PostScript and PDF output alongside the existing text, HTML, and XHTML outputs. This work was completed in August 2010. mandoc became the default formatter of manuals for OpenBSD 4.8, released in November 2010. It later became the default formatter in NetBSD, FreeBSD, illumos, Void Linux and Alpine Linux, and is also included in DragonFlyBSD and MINIX 3. Its advantages were stated as high speed, license, and clean reimplementation.
While mandoc works well with the mdoc manuals used in the BSD Operating Systems, it poorly handles older troff macros such as man, me, and the roff language itself. This makes it a poor fit for Linux distributions, which primarily use man troff macros and low-level roff for their manual pages.
mandoc has built-in support for the troff soelim preprocessor and partial built-in support for tbl and eqn. It has strong support for wide-character output. It can also use the semantic information in mdoc manuals along with sqlite to implement semantic search.