This is an implementation of the Kiwix offline Wikipedia reader as a progressive web app which runs in modern web browsers. You can download ZIM files through the app for later or open ones you already have. As long as it's an OpenZIM format archive file you can open it. When you install it, it shows up like any other application on the desktop.
Two modes: Jquery (for older browsers) and ServiceWorker (newer browsers, also supports archives with dynamic content)