> The easy way is decently secure: if you tick the checkbox, it memorises the certificate serial number for you, the same as if you had manually trusted a self-signed cert. Any change to the certificate will show the warning again. This is comparable to trusting an SSH certificate the first time you connect to a host with Putty.

Then it should say so.

"Click here to never show this again"—never show for this certificate? For this connection? For any connection?

Be clear and specific, even if it takes three more words.