How does this webpage load?

I was reading an article on my phone, when I decided to switch to reading on my computer. I opened my computer and used Airdrop to open the article directly in Safari on my Mac. At that point, the computer was still negotiating a wifi connection, so the page couldn’t load. This is pretty normal in my experience. Eventually wifi connects and then Safari will either automatically reload the page once it notices the connection, or I will manually reload the page. However, this time I was presented with a page that had loaded the actual website, and the website itself was giving me the message that I had no internet connection.

How does this work?

I’m not sure in this particular case, but I’d assume it’s something like you have visited the page in Safari on the Mac before, it was loading the page from cache, but the part that shows the actual content uses JavaScript to refresh the frame and that the JavaScript is clever enough to tell you when it can’t refresh due to connectivity.

I’m positive I’ve been on the site before, but I’m also positive I had never loaded the page before on my mac.