This site is currently being rebuilt in the background and the current major project is to switch to PWA during 2022 and this has proved to be more difficult than expected.
As of 2021, PWA features are supported to varying degrees by Google Chrome, Apple Safari, Firefox for Android, and Microsoft Edge but not by Firefox for desktop.
Since a progressive web app is a type of webpage or website known as a web application, they do not require separate bundling or distribution. Developers can just publish the web application online, ensure that it meets baseline "installability requirements", and users will be able to add the application to their home screen. Publishing the app to digital distribution systems like Apple App Store or Google Play is optional.
In 2015, designer Frances Berriman and Google Chrome engineer Alex Russell coined the term "progressive web apps" to describe apps taking advantage of new features supported by modern browsers, including service workers and web app manifests, that let users upgrade web apps to progressive web applications in their native operating system (OS). Google then put significant efforts into promoting PWA development for Android. Firefox introduced support for service workers in 2016, and Microsoft Edge and Apple Safari followed in 2018, making service workers available on all major systems.
By 2019, PWAs were supported by desktop versions of most browsers, including Microsoft Edge (on Windows) and Google Chrome (on Windows, macOS, Chrome OS and Linux).
In December 2020, Firefox for desktop abandoned implementation of PWAs (specifically, removed the prototype "site-specific browser" configuration that had been available as an experimental feature). A Firefox architect noted: "The signal I hope we are sending is that PWA support is not coming to desktop Firefox anytime soon." Mozilla still plans to support PWAs on Android.
Since a progressive web app is a type of webpage or website known as a web application, they do not require separate bundling or distribution. In particular, there is no requirement for developers or users to install the web apps via digital distribution systems like Apple App Store, Google Play, Microsoft Store or Samsung Galaxy Store. To varying degrees, the major app stores support publication of PWAs. Google Play, Microsoft Store, and Samsung Galaxy Store support PWAs, but Apple App Store does not. Microsoft Store publishes some qualifying PWAs automatically (even without app authors' request) after discovering them via Bing indexing.
Progressive web apps are all designed to work on any browser that is compliant with the appropriate web standards. As with other cross-platform solutions, the goal is to help developers build cross-platform apps more easily than they would with native apps. Progressive web apps employ the progressive enhancement web development strategy.
Some progressive web apps use an architectural approach called the App Shell Model. In this model, service workers store the Basic User Interface or "shell" of the responsive web design web application in the browser's offline cache. This model allows for PWAs to maintain native-like use with or without web connectivity. This can improve loading time, by providing an initial static frame, a layout or architecture into which content can be loaded progressively as well as dynamically.
The technical baseline criteria for a site to be considered a progressive web app and therefore "installable" by browsers were described by Russell in a follow-up post and updated since:
Originate from a secure origin. Served over TLS and green padlock displays (no active mixed content). Progressive web apps must be served via HTTPS to ensure user privacy, security, and content authenticity.
Register a service worker with a fetch handler. Progressive web apps must use service workers to create programmable content caches. Unlike regular HTTP web cache, which caches content after the first use and then relies on various heuristics to guess when content is no longer needed, programmable caches can explicitly prefetch content in advance before it's used for the first time and explicitly discard it when it is no longer needed. This requirement helps pages to be accessible offline or on low quality networks.
Reference a web app manifest. The manifest must contain at least the five key properties: name or short_name, start_url, and display (with a value of standalone, fullscreen or minimal-ui), and icons (with 192px and a 512px versions). Information contained in manifest makes PWAs easily shareable via a URL, discoverable by a search engines, and alleviates complex installation procedures (but PWAs still can be listed in a third-party app store). Furthermore, PWAs support native app-style interactions and navigation, including being added to home screen, displaying splashscreens, etc.