Progressive web application

A progressive web application (PWA) is a type of application software delivered through the web, built using common web technologies including HTML, CSS and JavaScript. It is intended to work on any platform that uses a standards-compliant browser, including both desktop and mobile devices.

PWA logo

PWA features narrow the gap between user experience in web-based and native applications.

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 or Google Play. However, for many PWA providers the lack of visibility of the PWA in those distribution systems is a disadvantage compared to native apps. To varying degrees, the major app stores started to embrace PWAs, allowing them to be found in app stores.[1]

As of 2021, the PWA-related features are supported to varying degrees by the Google Chrome, Apple Safari, Firefox for Android, and Microsoft Edge browsers.[2][3] Several businesses highlight significant improvements[4] in a wide variety of key performance indicators after PWA implementation, like increased time spent on page, conversions, or revenue.

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."[5] Mozilla still plans to support PWAs on Android.[6]

Browser support
Browser Support Comment
Windows Linux macOS Android iOS
Google Chrome Yes Yes Yes Yes Yes
Firefox No[5] No[5] No Partial No
Safari N/A N/A Yes N/A iOS 11.3+[7]

Background

At the launch of the iPhone in 2007, Steve Jobs announced that web apps, developed in HTML5 using AJAX architecture, would be the standard format for iPhone apps. No software development kit (SDK) was required, and the apps would be fully integrated into the device through the Safari browser engine.[8] This model was later switched for the App Store, as a means of preventing jailbreakers and of appeasing frustrated developers.[9] In October 2007 Jobs announced that an SDK would be launched the following year.[8] As a result, although Apple continued to support webapps, the vast majority of iOS applications shifted towards the App Store.

Beginning in the early 2010s dynamic web pages allowed web technologies to be used to create interactive web applications. Responsive web design, and the screen-size flexibility it provides, made PWA development more accessible. Continued enhancements to HTML, CSS, and JavaScript allowed web applications to incorporate greater levels of interactivity, making native-like experiences possible on a website, and therefore on PWAs.[10]

Firefox released Firefox OS in 2013. It was intended to be an open-source operating system for running webapps as native apps on mobile devices, with Gaia built as its HTML5 interface. The development of Firefox OS ended in 2016.

In 2015, designer Frances Berriman and Google Chrome engineer Alex Russell coined the term "progressive web apps"[11] 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.[12][13] With Apple's introduction of service worker support for Safari in 2017,[12] PWAs were now supported on the two most commonly-used mobile operating systems, Android and iOS.

By 2019, PWAs were available on desktop browsers Microsoft Edge[14] (on Windows) and Google Chrome[15] (on Windows, macOS, Chrome OS and Linux).

Characteristics

Progressive web apps are 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.[12] According to Google Developers,[11][16][17] the characteristics of a PWA are:

  • Progressive — Works for every user, regardless of browser choice, using progressive enhancement principles.
  • Responsive — Fits any form factor: desktop, mobile, tablet, or forms yet to emerge.
  • Faster after initial loading - After the initial loading has finished, the same content and page elements do not have to be re-downloaded each time.
  • Connectivity independent — Service workers allow offline uses, or on low quality networks.
  • App-like — Feels like an app to the user with app-style interactions and navigation.
  • Fresh — Always up-to-date due to the service worker update process.
  • Safe — Served via HTTPS to prevent snooping and ensure content hasn't been tampered with.
  • Discoverable — Identifiable as an “application” by manifest.json[18] and service worker registration, and discoverable by search engines.
  • Re-engageable — Ability to use push notifications to maintain engagement with the user.
  • Installable — Provides homescreen icons without the use of an App Store.
  • Linkable — Can easily be shared via a URL, and does not require complex installation.

The technical baseline criteria for a site to be considered a progressive web app by browsers were described by Russell in a follow-up post:[19]

  • Originate from a secure origin. Served over TLS and green padlock displays (no active mixed content).
  • Load while offline (even if only a custom offline page). By implication, this means that progressive web apps require service workers.
  • Reference a web app manifest with at least the four key properties: name, short_name, start_url, and display (with a value of standalone or fullscreen)
  • An icon at least 144×144 large in PNG format.
  • Use of vector graphics that are indefinitely scalable and require smaller file sizes.

Some progressive web apps use an architectural approach called the App Shell Model.[20] 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.[21]

Comparison with native apps

In 2017, Twitter released Twitter Lite, a PWA alternative to the official native Android and iOS apps. According to Twitter, Twitter Lite consumed only 1-3% of the size of the native apps.[22]

Technologies

There are many commonly used technologies to create progressive web apps. All PWAs require at minimum a service worker and a manifest.[23][24][25]

Manifest

The web app manifest[26] is a W3C specification defining a JSON-based manifest (usually labelled manifest.json)[18] to provide developers a centralized place to put metadata associated with a web application including:

  • The name of the web application
  • Links to the web app icons or image objects
  • The preferred URL to launch or open the web app
  • The web app configuration data
  • Default orientation of the web app
  • The option to set the display mode, e.g. full screen

This metadata is crucial for an app to be added to a home screen or otherwise listed alongside native apps.

iOS support

iOS Safari partially implements manifests, while most of the PWA metadata can be defined via Apple-specific extensions to the meta tags. These tags allow developers to enable full-screen display, define icons and splash screens, and specify a name for the application.[27][28]

WebAssembly

WebAssembly allows precompiled code to run in a web browser, at near-native speed.[29] Thus, libraries written in languages such as C can be added to web apps. Due to the cost of passing data from JavaScript to WebAssembly, near-term uses will be mainly number-crunching (such as voice recognition and computer vision), rather than whole applications.

Data storage

Progressive Web App execution contexts get unloaded whenever possible, so progressive web apps need to store majority of long-term internal state (user data, dynamically loaded application resources) in one of the following manners

Web Storage
Main article: Web storage

Web Storage is a W3C standard API that enables key-value storage in modern browsers. The API consists of two objects, sessionStorage (that enables session-only storage that gets wiped upon browser session end) and localStorage (that enables storage that persists across sessions).[30]

Service workers

A service worker is a web worker that implements a programmable network proxy that can respond to web/HTTP requests of the main document. It is able to check the availability of a remote server and to cache content when that server is available, and serve that content later to the document. Service workers, like any other web workers, work separately from the main document context. Service workers can handle push notifications and synchronize data in the background, cache or retrieve resource requests, intercept network requests and receive centralized updates independently of the document that registered them, even when that document is not loaded.[31]

Service workers go through a three-step lifecycle of Registration, Installation and Activation. Registration involves telling the browser the location of the service worker in preparation for installation. Installation occurs when there is no service worker installed in the browser for the webapp, or if there is an update to the service worker. Activation occurs when all of the PWAs pages are closed, so that there is no conflict between the previous version and the updated one. The lifecycle also helps maintain consistency when switching among versions of service worker since only a single service worker can be active for a domain.[31]

Indexed Database API
Main article: Indexed Database API

Indexed Database API is a W3C standard database API available in all major browsers. The API is supported by modern browsers and enables storage of JSON objects and any structures representable as a string.[32] Indexed Database API can be used with a wrapper library providing additional constructs around it.

AppCache (obsolete)
Main article: Cache manifest in HTML5

Application Cache (or AppCache or HTML5 cache manifest) is an earlier technology that allowed the application to cache content in advance for later use when the device is offline.[33] It works adequately for single-page applications which it was designed for, but fails in problematic ways for multi-page applications like wikis.[34] As of May 2019, the technology is supported by major browsers and in use for years by some sites, but it is already deprecated in favor of service workers and will eventually be removed.

See also

References
  1. "Progressive Web Apps | Software AG". techradar.softwareag.com. Retrieved 2020-09-25.
  2. "Can I use pwa?". CanIUse. Retrieved 27 January 2021.
  3. "Is Service Worker Ready?". Jake Archibald.
  4. Rzutkiewicz, Jason. "Why Progressive Web Apps Are The Future of Mobile Web (2019 Research)". YML. Retrieved 4 October 2019.
  5. Newman, Jared (2021-01-26). "Firefox just walked away from a key piece of the open web". Fast Company. Retrieved 2021-01-27.
  6. agi90 (19 December 2020). "Comment". Reddit. We have no plans of sunsetting PWAs on mobile that I know of.
  7. Firtman, Maximiliano (2020-08-18). "Progressive Web Apps on iOS are here 🚀". Medium. Retrieved 2021-01-29.
  8. Ritchie, Rene (5 March 2018). "App Store Year Zero: Unsweet web apps drove iPhone to an SDK". iMore. Retrieved 23 May 2019.
  9. "Jobs' original vision for the iPhone: No third-party native apps". 9to5Mac. 21 October 2011. Retrieved 22 May 2019.
  10. Marcotte, Ethan. "Responsive Web Design". A List Apart. Retrieved May 25, 2010.
  11. Russell, Alex. "Progressive Web Apps: Escaping Tabs Without Losing Our Soul". Retrieved June 15, 2015.
  12. Evans, Jonny (26 January 2018). "Apple goes back to the future with web apps". Computerworld. Retrieved 23 May 2019.
  13. Ladage, Aaron (17 April 2018). "Progressive Web Apps Are Here and They're Changing Everything". DEG. Retrieved 23 May 2019.
  14. Pflug, Kyle (6 February 2018). "Welcoming Progressive Web Apps to Microsoft Edge and Windows 10". Microsoft Windows Blogs. Retrieved 13 September 2019.
  15. LePage, Pete (4 June 2019). "Progressive Web Apps on Desktop". Google Developers. Retrieved 13 September 2019.
  16. "Your First Progressive Web App | Web Fundamentals - Google Developers". Your First Progressive Web App | Web Fundamentals - Google Developers. Retrieved 2016-07-17.
  17. Google Developers. "Progressive Web App". Retrieved June 15, 2015.
  18. W3C “Web App Manifest”, Working Draft, retrieved 12 September 2016
  19. Russell, Alex. "What, Exactly, Makes a Progressive Web App". Retrieved October 18, 2016.
  20. "The App Shell Model".
  21. Osmani, Addi. "The App Shell Model | Web Fundamentals". Google Developers. Retrieved 23 May 2019.
  22. "How we built Twitter Lite". blog.twitter.com. Retrieved 2021-01-27.
  23. "Discoverable". Mozilla Developer Network. Retrieved 2017-04-24.
  24. "Network independent". Mozilla Developer Network. Retrieved 2017-04-24.
  25. "Instant Loading Web Apps with an Application Shell Architecture". Google Developers. Retrieved 2017-04-24.
  26. "Web Manifest Docs on MDN". MDN Web Docs.
  27. "What's new on iOS 12.2 for Progressive Web Apps". Medium. 27 March 2019.
  28. "Configuring Web Applications". Safari Web Content Guide.
  29. "WebAssembly Concepts". MDN. Retrieved 14 August 2018.
  30. "Web Storage API". MDN. Retrieved 14 August 2018.
  31. "Introduction to Service Worker | Web". Google Developers. 1 May 2019. Retrieved 23 May 2019.
  32. "Concepts behind IndexedDB". MDN. Retrieved 14 August 2018.
  33. "Using the application cache". MDN. Mozilla. Retrieved 14 August 2018.
  34. "Application Cache is a Douchebag". A List Apart. Retrieved 14 August 2018.
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