Ok it seems I have found a quick way to solve all of these problems. For the records, I note it down here:
Add the following global directive to /etc/httpd/conf/httpd.conf (for the global Apache) and to /root/ispconfig/httpd/conf/httpd.conf (for ISPConfig's Apache to port 81).
Restart your global Apache:
and ISPConfig's Apache:
killall -1 ispconfig_httpd
Everything should display just fine now!
However what I haven't tested yet, is if an html document can override this setting (either with a metatag inside the html or with another directive in .htaccess or in ISPConfig's specific Apache options). If anyone knows, please share.
Thank you all for reading.