Ubuntu 14.04 provided modapache2_mod_proxy_html as an option to pass requests to Apache through to something like NodeJS running on a different port. However, this package does not exist for 16.04!
That’s okay, though. Because it’s already there. You just need to enable it.
sudo a2enmod proxy
sudo a2enmod proxy_http
sudo a2enmod proxy_html
Then restart the service.
sudo service apache2 restart
Voila! Enable and go, no install required!