This website uses service workers. Chris Love explains service workers thusly:
Service workers are this wonderful new web platform feature that lights up some great functionality like URL response caching. This caching is the magic that enables progressive web applications to work offline.
The idea behind service workers is to speed up loading of this website and make past pages available offline. The first part was accomplished by caching before—and still is—but caching gives developers poor control and promises weak persistence of data. Service workers are generally more reliable. The second part is unique to service workers: if you visit a bunch of pages on this website or read a handful of articles, those articles will be available for you to view even when you are offline. Try it now: turn off your internet connection and refresh this page and it will load just fine.
Service workers do not make this site available offline eternally, however. In the interest of updating changes and the possibility that you have lost interest in this site nothing is stored for long after you stop visiting this site. In short, service workers improve your experience on this site without harming your system in any way or invading your privacy.