Published: June 15, 2020
When you play Progressive Web App buzzword bingo, it's safe to bet on "PWAs are just websites." Microsoft's PWA documentation agrees, we say it, and so have PWA nominators Frances Berriman and Alex Russell. Yes, PWAs are just websites, but they are also way more than that. If done right, a PWA won't feel like a website, but like a real app.
But what does it mean to feel like a real app?
Take the Apple Podcasts app as an example. It's available on macOS on desktop and on iOS (and iPadOS respectively) on mobile. While Podcasts is a media application, the core ideas I illustrate with its help apply to other categories of apps, too.
Capable of running offline
Think about the platform-specific applications you have on your mobile phone or desktop. One thing stands out: you can always use it. In the Podcasts app, even if I'm offline, there is always something to do or see. When there is no network connection, the app still opens. The Top Charts section doesn't show any content, but instead, it falls back to a Can't connect right now message paired with a Retry button.
Do this on the web
The Podcasts app follows the so-called app shell model. All the static content that is needed to show the core app is cached locally, including decorative images like the left-hand menu icons and the core player UI icons. Dynamic content like the Top Charts data is only loaded on demand, with locally cached fallback content available should the loading fail. Read the article The App Shell Model to learn how to apply this architectural model to your web app.
Offline content available and media playable
While offline, in the left-hand drawer, I can still navigate to the Downloaded section and play downloaded podcast episodes, which are displayed with all metadata, including artwork and descriptions.
Do this on the web
Previously downloaded media content can be served from the cache, for example using the Serve cached audio and video recipe from the Workbox library. Other content can always be stored in the cache, or in IndexedDB. Read the article Storage for the web for all details and to know when to use what storage technology. If you have data that should be persistently stored without the risk of being purged when the available amount of memory gets low, you can use the Persistent Storage API.
Proactive background downloading
When I am back online, I can of course search for content with a query like http 203, and when I decide to subscribe to the search result, the HTTP 203 podcast, the latest episode of the series is immediately downloaded, no questions asked.
Do this on the web
Downloading a podcast episode is an operation that potentially can take longer. The Background Fetch API lets you delegate downloads to the browser, which takes care of them in the background.
On Android, the browser in turn can even delegate these downloads further on to the operating system, so the browser does not need to be continuously running. Once the download has completed, your app's service worker gets woken up and you can decide what to do with the response.
Interact and share with other applications
The Podcasts app integrates naturally with other applications. For example, when I right-click an episode that I like, I can share it to other apps on my device, like the Messages app. It also naturally integrates with the system clipboard. I can right-click any episode and copy a link to it.
Do this on the web
The Web Share API and the Web Share Target API allow your app to share and receive texts, files, and links to and from other applications on the device. Although it's not yet possible for a web app to add menu items to the operating system's built-in right-click menu, there are lots of other ways to link to and from other apps on the device. With the Async Clipboard API, you can programmatically read and write text and image data (PNG images) to the system clipboard. On Android, you can use the Contact Picker API to select entries from the device's contacts manager. If you offer both a platform-specific app and a PWA, you can use the Get Installed Related Apps API to check if the platform-specific app is installed, in which case you don't need to encourage the user to install the PWA or accept web push notifications.
Background app refreshing
In the Podcasts app's settings, I can configure the app to download new episodes automatically. Like that, I don't even have to think about it, updated content will always just be there. Magic.
Do this on the web
The Periodic Background Sync API allows your app to refresh its content regularly in the background without the need for it to be running. This means new content is proactively available, so your users can start delving into it right away whenever they decide.
State synchronized over the cloud
My subscriptions are synchronized across all of my devices. In a seamless world, I don't have to worry about manually keeping my podcast subscriptions in sync. Likewise, I don't have to be afraid that my mobile device's memory will be consumed by episodes I've already listened to on my desktop. The play state is kept in sync, and listened-to episodes are automatically deleted.
Do this on the web
Syncing app state data is a task that you can delegate to the Background Sync API. The sync operation itself does not have to happen immediately, just eventually, and maybe even when the user has closed the app again already.
Hardware media key controls
When I am busy with another application, say, reading a news page in my browser, I can still control the Podcasts app with the media keys on my laptop. There is no need to switch to the app just to skip forward or backward.
Do this on the web
Media keys are supported by the Media Session API. Like that, users can make use of the hardware media keys on their physical keyboards, headphones, or even control the web app from the software media keys on their smartwatches. Additionally, you could use a vibration pattern when the user seeks over a significant part of the content, such as passing the opening credits or a chapter boundary.
Multitasking and app shortcut
Of course I can always multitask back to the Podcasts app from anywhere. The app has a clearly distinguishable icon that I can also put on my desktop or application dock so Podcasts can be launched immediately when I feel like it.
Do this on the web
Progressive Web Apps on both desktop and mobile can be installed to the home screen, start menu, or application dock. Installation can happen based on a proactive prompt, or fully controlled by the app developer. The article What does it take to be installable? covers everything you need to know. When multitasking, PWAs appear independent from the browser.
Quick actions in context menu
The most common app actions, Search for new content and Check for New Episodes, are available right from the context menu of the app in the Dock. In the Options menu, I can also decide to open the app at login time.
Do this on the web
By specifying app icon shortcuts in the PWA's web app manifest, you can register quick routes to common tasks that users can reach directly from the app icon. On operating systems like macOS, users can also right-click the app icon and set the app to launch at login time. There is ongoing work on a proposal for run on login.
Act as default app
Other iOS applications and even websites or emails can integrate with the Podcasts app by leveraging the podcasts:// URL scheme. If I follow a link like podcasts://podcasts.apple.com/podcast/the-css-podcast/id1042283903 while in the browser, I am brought right into the Podcasts app and can decide to subscribe or listen to the podcast.
Do this on the web
Handling fully custom URL schemes is not yet possible, but there's an
ongoing proposal for URL Protocol Handling
for PWAs. registerProtocolHandler() with a
web+ scheme prefix is the best alternative.
Local file system integration
You may not immediately think of it, but the Podcasts app naturally integrates
with the local file system. When I download a podcast episode to my laptop, it's
stored in ~/Library/Group Containers/243LU875E5.groups.com.apple.podcasts/Library/Cache.
Unlike, say ~/Documents, this directory is not meant to be accessed directly
by regular users.
Other storage mechanisms than files are referenced in the offline content section.
Do this on the web
The File System Access API enables developers to get access to the local file system of the device. You can use it directly or using the browser-fs-access support library that transparently provides a fallback for browsers that don't support the API. For security reasons, system directories are not web-accessible.
Platform look and feel
There is a more subtle feature that is self-evident for an iOS application, like Podcasts. None of the text labels are selectable and all text blends in with the system font of the machine. Also, my choice of system color theme (such as dark mode) is respected.
Do this on the web
By leveraging the user-select
CSS property with the value of none,
you can protect UI elements from being accidentally selected.
Be sure, though, to not abuse this property for making app contents unselectable.
It should only be used for UI elements like button texts, etc.
The system-ui
value for the font-family CSS property lets you
specify the default UI font of the system to be used for your app.
Finally, your app can obey to the user's color scheme preference by respecting their prefers-color-scheme choice, with an optional dark mode toggle
to override it.
Another thing to decide on might be what the browser should do when reaching
the boundary of a scrolling area, for example, to implement custom pull to refresh.
This is possible with the overscroll-behavior CSS property.
Customized title bar
When you look at the Podcasts app window, you notice that it does not have a classic integrated title bar and toolbar, like, for example, the Safari browser window, but a customized experience that looks like a sidebar docked to the main player window.
Do this on the web
Title bar customization
has limited availability. You can (and should) specify the
display and the
theme-color
properties of the web app manifest. This determines the look and feel of
your application window and decides which default browser controls should
be shown—potentially none.
Snappy animations
In-app animations are snappy and smooth in Podcasts. For example, when I open the Episode Notes drawer on the right, it elegantly slides in. When I remove one episode from my downloads, the remaining episodes float up and consume the screen real estate that was freed by the deleted episode.
Do this on the web
Performant animations on the web are certainly possible if you take into account a number of best practices outlined in the article Animations and Performance. Scroll animations as commonly seen in paginated content or media carousels can be massively improved by using the CSS Scroll Snap feature. For full control, you can use the Web Animations API.
Content surfaced outside of app
The Podcasts app on iOS can surface content in other locations than the actual application, for example, in the system's Widgets view or as a Siri Suggestion.
Proactive, usage-based calls-to-action that just require a tap to interact with can greatly increase the re-engagement rate of an app like Podcasts.
Do this on the web
Use the Content Index API in your application to tell the browser what content of the PWA is available offline. This allows the browser to expose this content outside of the main app. By marking up interesting content in your app as suitable for speakable audio playback and by using structured markup, you can help search engines and virtual assistants, like the Google Assistant, present your offerings in an ideal light.
Lock screen media control widget
When a podcast episode is playing, the Podcasts app shows a beautiful control widget on the lock screen that features metadata like the episode artwork, the episode title, and the podcast name.
Do this on the web
The Media Session API lets you specify metadata like artwork, track titles, etc. that then gets displayed on the lock screen, smartwatches, or other media widgets in the browser.
Push notifications
Push notifications have become a bit of an annoyance on the web (albeit notification prompts are a lot quieter now). But if used properly, they can add a lot of value. For example, the iOS Podcasts app can optionally notify me of new episodes of podcasts I am subscribed to or recommend new ones, as well as alert me of new app features.
Do this on the web
The Push API allows your app to receive push notifications so you can notify your users about noteworthy events around your PWA. For notifications that should fire at a known time in the future and that don't require a network connection, you can use the Notification Triggers API.
App icon badging
Whenever there are new episodes available for one of the podcasts I am subscribed to, an app icon badge on the Podcasts home screen icon appears, again encouraging me to re-engage with the app in a way that is not intrusive.
Do this on the web
You can set app icon badges with the Badging API. This is especially useful when your PWA has some notion of "unread" items or when you need a means to unobtrusively draw the user's attention back to the app.
Media playback takes precedence over energy saver settings
When podcast media is playing, the screen may turn off, but the system doesn't enter standby mode. Apps can optionally keep the screen awake, too, for example to display lyrics or captions.
Do this on the web
Use the Screen Wake Lock API to prevent the screen from turning off. Media playback on the web automatically prevents the system from entering standby mode.
App discovery through an app store
While the Podcasts app is part of the macOS desktop experience, on iOS it needs
to be installed from the App Store. A quick search for podcast, podcasts,
or apple podcasts immediately turns the app up in the App Store.
Do this on the web
While Apple doesn't allow PWAs on the App Store, with Android, you can
submit your PWA wrapped in a
Trusted Web Activity.
The bubblewrap
script makes this a painless operation. This script is also what internally
powers PWABuilder's Android app
export feature, which you can use without touching the command line.
Feature summary
This table shows a compact overview of all features and provides a list of useful resources for realizing them on the web.
Conclusion
PWAs have come a long way since their introduction in 2015. In the context of Project Fugu 🐡, the cross-company Chromium team is working on closing the last remaining gaps. By following even only some of the pieces of advice we've shared, you can get closer to that app-like feeling, and make your users forget that they are dealing with "just a website." Ultimately, most users don't care how your app is built (and why should they?), as long as it feels like a real app.
Acknowledgements
This document was reviewed by Kayce Basques, Joe Medley, Joshua Bell, Dion Almaer, Ade Oshineye, Pete LePage, Sam Thorogood, Reilly Grant, and Jeffrey Yasskin.