This checks the certificate this server is actually presenting right now — read
directly, server-side, over a local connection that never leaves this machine, so it can't
itself be intercepted by whatever network you're on.
How to actually use this: click the padlock/lock icon in your browser's
address bar → Certificate (or "Connection is secure" → Certificate is valid) → look for the
SHA-256 fingerprint. Compare it, character for character, against the fingerprint shown below.
If they don't match, something between you and this server is intercepting your traffic —
close the tab and don't log in on this network.