Emacs's nxml-mode uses compact relax NG schemas for validating XML. A compact relax NG schema for FreeBSD's extension to DocBook 5.0 is included in the documentation repository. To configure nxml-mode to validate using this schema, create ~/.emacs.d/schema/schemas.xml and add these lines to the file:

<locatingRules xmlns="http://thaiopensource.com/ns/locating-rules/1.0">
  <documentElement localName="section" typeId="DocBook">
  <documentElement localName="chapter" typeId="DocBook">
  <documentElement localName="article" typeId="DocBook">
  <documentElement localName="book" typeId="DocBook">
  <typeId id="DocBook" uri="/usr/local/share/xml/docbook/5.0/rng/docbook.rnc">
</locatingRules>

The Flycheck package is available from Milkypostman's Emacs Lisp Package Archive (MELPA). If MELPA is not already in Emacs's packages-archives, it can be added by evaluating

(add-to-list 'package-archives '("melpa" . "http://stable.melpa.org/packages/") t)

Add the line to Emacs's initialization file (one of ~/.emacs, ~/.emacs.el, or ~.emacs.d/init.el) to make this change permanent.

To install Flycheck, evaluate

(package-install 'flycheck)

Create a Flycheck checker for textproc/igor by evaluating

(flycheck-define-checker igor
  "FreeBSD Documentation Project sanity checker.

See URLs https://www.freebsd.org/docproj/ and
http://www.freshports.org/textproc/igor/."
  :command ("igor" "-X" source-inplace)
  :error-parser flycheck-parse-checkstyle
  :modes (nxml-mode)
  :standard-input t)

  (add-to-list 'flycheck-checkers 'igor 'append)

Again, add these lines to Emacs's initialization file to make the changes permanent.

To apply settings specific to the FreeBSD documentation project, create .dir-locals.el in the root directory of the documentation repository and add these lines to the file:

;;; Directory Local Variables
;;; For more information see (info "(emacs) Directory Variables")

((nxml-mode
  (eval . (turn-on-auto-fill))
  (fill-column . 70)
  (eval . (require 'flycheck))
  (eval . (flycheck-mode 1))
  (flycheck-checker . igor)
  (eval . (add-to-list 'rng-schema-locating-files "~/.emacs.d/schema/schemas.xml"))))