* asciidoc >= 8.3.0 for docs/hacking-howto
* asciidoc, xmlto, docbook-xml for man/i3.man
* Xlib, the one that comes with your X-Server
+ * x11-utils for xmessage (only for displaying the welcome message, so this is
+ mainly interesting for distributors)
Recommendations:
* i3lock for locking your screen