Why do some browsers not accept the security certificate?

Question

HTML

There is a website: https://lesson-web.ru/
As a rule, everything opens well. But it happens (especially often from mobile devices), if I go to the site, it gives out such a thing:

Why sometimes browsers do not perceive the site as safe?

Answer 1

The list of accredited certificate authorities is initially loaded into the browser, and if a certificate is returned that is not signed by anyone from this list, then you receive this warning. Accordingly, your browser does not have your CA listed

CAs are companies that perform manual verification that a person attempting to obtain a certificate satisfies the following two conditions:
1. is a real person;
2. has access to the domain for which it is trying to obtain a certificate

Answer 2

https requires a certificate on the server. It may not exist at all, or it may be expired or not valid. So there are many reasons. I noticed that apple devices are especially nervous about certificates, so if you log in from a bitten apple, redirects and magnetic storms, bad mood of the developers of your firmware version, etc. will add to the list of reasons. On the rest of the devices, which were made not for the sake of "sell more expensive", but for the sake of "what would work" - only the first three options.

Answer 3

The mobile operator sniffs your traffic. Unfortunately, this has become the norm in recent years.