The encryption certificate is only valid for gitgud.io, not www. gitgud.io (note: I had to insert a small space to prevent F95 from automatically linking to the wrong address). But the same certificate is used for both addresses. If your browser tries to go to www. gitgud.io, it sees a certificate that does not match the exact name of the site that you requested. This will trigger a warning in Firefox. If you proceed despite the warning, then you will see that www. gitgud.io redirects you to the correct address gitgud.io without the www.
If you want to check the certificate for gitgud.io in Firefox, you can do that by clicking on the padlock icon next to the address, then clicking on "Connection secure" > "More information". You can then click on the button "View certificate". The first section "Subject Name" will show you that the Common Name is only for gitgud.io. The section "Subject Alt Names" also shows the DNS Name gitgud.io. Alternatively, you can also use a certificate validation tool such as
You must be registered to see the links
and you will see a similar information presented differently.
Now if you go to www. gitgud.io, you will also see the exact same certificate that is only valid for gitgud.io, not www. gitgud.io. This causes your browser to complain because the domain name included in the certificate does not match the domain name that you requested.