1) Load the page.
2) The page is now fully loaded and ready for reading without any need for further network requests.
For even more offline use, try printing it! HN threads work superbly when printed, again unlike most modern sites.
Or you could reimplement newsgroups (but with less interoperability) as an SPA, which in theory would work great (once you work out the kinks) but in practice would suffer from all kinds of little issues in the from-scratch reimplementation of everything. For example, printing would probably forever be a wishlist item in the backlog.
A lot of people think that moving to an SPA would mean killing both those aspects. But the truth is, you CAN have an SPA that is both lightweight and fast - the developer just needs to know what they're doing.