How to display a stub no internet connection on the site?

Question

Network administration

in general, I came across this at aviasales.ru

, if I previously visited the site it is displayed, that is, if I now go to aviasales and then turn off the Internet and go, it gives such an error.

But when the Internet is turned off, open in incognito, the browser error is already displayed and not this stub.

How are moments like this set up?

Answer 1

Yes, go through the Service Worker
PS it is https://www.aviasales.ru/service-worker-pwa.js

Answer 2

This is the site code. When writing a site, a function is made that checks the availability of the web server.
If the Web server does not respond, then the site displays a stub.