Illarion.org

In the rare case when you want to access an illarion https page, you might notice, that your browser does not consider our certificate to be trustworthy. This is due to the fact, that we use CAcert as signing authority and that their root certificate is probably not included in your browser. If you encounter this problem, you might want to include their root certificate.

In layman's terms, if you get a warning about the trustworthiness of a certificate while browsing Illarion's webpage, Vilarion assures you it's perfectly safe to approve access to the page.

Actually that is not quite what I meant. If you accept the certificate, you will need to accept it again every time it changes, which is at least every two years but might be more often.
If you install the mentioned root certificates, then there will be no more warnings about illarion.org in your browser. So this is a more permanent solution and recommended.