You type an old address beginning with HTTP, yet your browser goes straight to HTTPS. A redirect may be involved, but a remembered security policy called HSTS can also tell the browser to use the encrypted connection before making an ordinary HTTP request.
HSTS stands for HTTP Strict Transport Security. Mozilla’s header documentation explains how a site sends the policy over HTTPS and tells a browser how long to remember it. The policy is associated with a host name, not with a judgement that every page on that host is trustworthy.
Why a certificate error can feel different
For an HSTS host, browsers do not offer the usual option to bypass certain certificate errors and continue. That is part of the protection, not necessarily evidence that the browser has broken.
If an error appears, check the address and your device’s date and time, and seek the site’s legitimate support route if needed. Do not follow an unsolicited message asking you to install a certificate or disable browser security as a quick fix.
What happens on the first visit?
A browser normally has to learn the policy from a secure response. That leaves a distinction between a first encounter and later visits. Mozilla’s TLS guidance discusses the initial HTTP-to-HTTPS upgrade problem. Preloaded HSTS lists can address that first-contact gap for included hosts, but not every HTTPS site is preloaded.
You do not need to inspect the list every time you browse. The practical habit is to use the correct HTTPS address and take connection warnings seriously.
Encryption is one part of trust
A fraudulent site can also use HTTPS. HSTS protects how a browser connects to a named site; it does not verify an offer, establish the identity of a person emailing you or guarantee that a download is harmless.
Before entering information, check the domain itself and why the site is asking for it. Treat “the connection is protected” and “this is the organisation I intended to visit” as separate questions. The browser can help with the first while still needing your attention on the second.