El Capitan Safari can't open http://localhost/ after starting Apache
Turns out that there is evidence that upgrading from Snow Leopard to El Capitan upgrades the apache2 binaries, but not its configuration. Please get the original apache2 config from a El Capitan clean-install and replace yours with it. It's located at /private/etc/apache2/original. Replace your httpd.conf and the folder extra with those you got from the clean install. Restart apache2 afterwards.
type
apachectl configtest
into your command line and see what errors you get...