localhost / userdir on macOS Sierra
There is an updated tutorial here: How to Set up Apache in macOS Sierra 10.12

It includes extra configurations like uncommenting the following lines by removing the "#":


  #LoadModule userdir_module libexec/apache2/mod_userdir.so
  ...
  #LoadModule vhost_alias_module libexec/apache2/mod_vhost_alias.so
  ...
  #Include /private/etc/apache2/extra/httpd-userdir.conf
  ...
  #Include /private/etc/apache2/extra/httpd-vhosts.conf


in /etc/apache2/httpd.conf

and uncommenting:


  #Include /private/etc/apache2/users/*.conf


in /etc/apache2/extra/httpd-userdir.conf

and restarting apache: sudo apachectl restart

It should work.