Is it safe to visit a website with an expired certificate?

Visiting a website with an expired certificate is usually not dangerous by itself — the encryption still works technically — but you've lost the guarantee that the site is who it claims to be, so treat it as unverified. Fine for casually reading a page; a hard no for logging in, paying, or entering personal data.

An expired certificate most often means simple negligence (someone forgot to renew), but the browser can't tell negligence from an attack, which is why it shows the scary interstitial either way.

If it's your own site showing the warning: this is a five-alarm business problem, not a cosmetic one — most visitors leave immediately. Renew the certificate, then fix the process: automate renewal where possible, and put every non-automatable certificate somewhere with an owner and reminders.