Nodm
nodm is an automatic display manager which automatically starts an X session at system boot. It is meant for devices like smartphones, but can be used on a regular computer as well, if the security implications are acceptable.
The author is no longer continuing to develop, and had written a minimal autologin greeter for lightdm (lightdm-autologin-greeter-gitAUR) instead. It has the same autologin behaviour as nodm, but being based on lightdm it stays on top of modern display manager requirements.
Usage
Now ensure no other display managers get started by disabling their systemd services.
After installing nodm, modify the /etc/nodm.conf file. Set the NODM_USER variable to the user which should be automatically logged in, and change the NODM_XSESSION variable to point to the script that starts your session. The NODM_XSESSION script must be executable!
/etc/nodm.conf
NODM_USER=''user'' NODM_XSESSION=/home/''user/.xinitrc
Enable nodm.service so nodm will be started on boot.
Login session
For proper session handling, create pam.d file with the following content:
Known issues
Unclean poweroff/reboot
Users have reported in this issue that upon poweroff/reboot, systemd will fail to kill nodm, resulting in a delayed poweroff until systemd hard kills the remaining processes.