Sorry, just to clarify: - The site certificates are presented *to* your browser *from* the website. They expire if the website operator fails to renew them. They can also be revoked (but this should have a different alert message). - Your browser trusts the site certificate if it was issued by a trusted certificate authority and has not expired. The trusted certificate authorties come from your browser/OS, not the website. Trusted certificate authorities also expire, so your browser/OS vendor must keep them up to date via software upgrades.
James.
