If you search for "Eaglercraft UPD download," you are likely looking for an offline HTML file or a specific fork. The beauty of Eaglercraft is that for 90% of users, You simply navigate to a website hosting the client, and it loads in the browser.
After first load (which downloads assets from the internet), the game caches everything in IndexedDB. You can then play offline for weeks. eaglercraft upd download chromebook