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Amberley Romo
Amberley Romo

Posted on • Originally published at Medium on

Seriously, though. What is a progressive web app?

“unofficial” community logo for PWA

I’ve been familiar with the “progressive web app” (PWA) concept for a while now. But there’s always been an itch in the back of my mind about it.

I’ve been to talks at conferences and meetups about the subject. I’ve read the main docs and top-level “what is a PWA” articles. I’ve read Alex Russell’s post, “Progressive Web Apps: Escaping Tabs Without Losing Our Soul” [Posted 06/15/2015], about the discussion between himself and Frances Berriman that originated the term, and Frances’ post, “Naming Progressive Web Apps” [Posted 06/26/2017], about the semantic foofaraw around the term “progressive web app” itself.

And still, I have found it difficult to conclusively, accurately, and concisely define it (based on concrete citation, that is). I see similar confusion across the internet.

One such example — Ben Halpern of, in a post called “What the heck is a “Progressive Web App”? Seriously.” [Posted 11/06/2017, updated 04/21/2018]:

This morning I went to write an article outlining a few tips about implementing a progressive web app (PWA). But when I went to introduce the topic, I again encountered what was the hardest part about the whole topic in the first place: I have a really hard time describing what a progressive web app actually is .

He captured a lot of my “is this just me?” feels too:

Perhaps PWA is meant to be inclusive of a lot of technical approaches, so they need to be vague. This might be okay if it were just the source docs that were this way, but I feel like alternative introductions and how-tos scattered across the web have latched on to this vague language and the confusion has proliferated.

It’s socially hard to say “hold on, I don’t understand any of that”, because you don’t know if everybody else just gets it and you’re the stupid one. In the PWA space, I really do feel like there’s a lot of people nodding along assuming everyone else understands what a PWA is and they must be missing something. You know, imposter syndrome. [emphasis added]

Another such example is the uncertainty acknowledged in this blog post from Microsoft [Posted 02/06/2018] — a top-page hit when Googling “what is a progressive web”:

The excitement about PWAs in the developer community is almost palpable — but amongst all that excitement, it can be hard to pin down a single, concise, authoritative definition of a “Progressive Web App.” [emphasis added]

What had me particularly confused, personally, is that parts of the internet have seemingly coalesced around three baseline requirements for what comprises a progressive web app — and none of them cite where it came from. It’s impressive to me to achieve that level of unofficial, broad accord, given zero citation — it just echoes across the internet. (And the worst part? I’m guilty; I did the same thing in “A Browser-Based, Open Source Tool for Alternative Communication” [Posted 03/14/2018]. And I remember feeling conflicted about it at the time).

Because this time I couldn’t bring myself to divert from this mental rabbit hole, I started looking into:

  1. The origin of the listing of attributes that constitute a PWA that are generally referenced in docs and posts,
  2. The origin of the three baseline requirements for a PWA that are generally referenced in docs and posts, and,
  3. The variation in definitions among the top-level search results for “progressive web app”

At the end, I’ll post my conclusion of a definition for PWA (for whatever that’s worth).

The general attributes that constitute a PWA

This part is conclusive. In 2015, Alex Russell introduced the term “progressive web app” online. He recounted a conversation between himself and Frances Berriman, in which they “enumerated the attributes of [a] new class of applications” based on the gradual and powerful evolution of modern browsers. Those attributes are, verbatim:

  • Responsive: to fit any form factor
  • Connectivity independent: Progressively-enhanced with Service Workers to let them work offline
  • App-like-interactions: Adopt a Shell + Content application model to create appy navigations & interactions
  • Fresh: Transparently always up-to-date thanks to the Service Worker update process
  • Safe: Served via TLS (a Service Worker requirement) to prevent snooping
  • Discoverable: Are identifiable as “applications” thanks to W3C Manifests and Service Worker registration scope allowing search engines to find them
  • Re-engageable: Can access the re-engagement UIs of the OS; e.g. Push Notifications
  • Installable: to the home screen through browser-provided prompts, allowing users to “keep” apps they find most useful without the hassle of an app store
  • Linkable: meaning they’re zero-friction, zero-install, and easy to share. The social power of URLs matters.

The three baseline criteria for a PWA

The three baseline criteria a site must fulfill in order to qualify as a PWA echo across the web. They are:

  • You need to be running under HTTPS.

  • You need a Web App Manifest.

  • You need a Service Worker.

This in particular is copied verbatim from Aaron Gustafson’s “Yes, That Web Project Should Be a PWA” [Posted 08/30/2017]. I asked Aaron where, specifically, that apparently canonical list of three originates (beyond the vague, general internet pseudo-consensus).

He pointed me to Jeremy Keith, where I re-read, among others, his post, “HTTPS + service worker + web app manifest = progressive web app”. In it, he cites an older post of his own, “Progressing the web”, where he said:

Literally any website can be a progressive web app:

  • switch over to HTTPS,

  • add a JSON manifest file with your metacrap, and

  • add a service worker.

Later on, in response to a tweet from Jason Grigsby asking the same question I’m asking now — “I agree with you on the three things that comprise a PWA, but as far as I can tell, you’re the first to declare it as such. Am I wrong about that?” He responded:

I was quite surprised by that. I always assumed that I was repeating the three ingredients of a progressive web app, not defining them. But looking through all the docs out there, Jason might be right. It’s surprising because I assumed it was obvious why those three things comprise a progressive web app — it’s because they’re testable.

So, nothing conclusive on the origin. (So far.)

Variation in definitions among the top-level search results for “progressive web app”

While on this bender, I thought I’d review the variations in definitions of the top five Google search results for “progressive web app”. And thank god I did because I found a gem (queue suspense).

1. Google Developers: Progressive Web Apps

Progressive Web Apps are user experiences that have the reach of the web, and are:

Reliable — Load instantly and never show the downasaur, even in uncertain network conditions.

Fast — Respond quickly to user interactions with silky smooth animations and no janky scrolling.

Engaging — Feel like a natural app on the device, with an immersive user experience.

2. Google Developers: Your First Progressive Web App

Progressive Web Apps are experiences that combine the best of the web and the best of apps. They are useful to users from the very first visit in a browser tab, no install required. As the user progressively builds a relationship with the app over time, it becomes more and more powerful. It loads quickly, even on flaky networks, sends relevant push notifications, has an icon on the home screen, and loads as a top-level, full screen experience.

This resource also lists the canonical list of attributes, but adds a tenth:

Progressive — Works for every user, regardless of browser choice because it’s built with progressive enhancement as a core tenet.

3. Wikipedia: Progressive Web Apps

Progressive Web Apps (PWAs) are web applications that are regular web pages or websites, but can appear to the user like traditional applications or native mobile applications. The application type attempts to combine features offered by most modern browsers with the benefits of a mobile experience.

The Wikipedia entry lists web manifest and service workers as “commonly used technologies” to support the defined characteristics of a PWA (which it does cite to Berriman and Russell).

Then, lo, buried in a footnote, the Wikipedia entry attributes the technical baseline criteria for a PWA to none other than (🥁) Alex Russell, from his post: “What, Exactly, Makes Something A Progressive Web App?” [Posted 09/12/2016]. (This is the gem, by the way 💎).

This is surprising to me for a couple reasons. One, how had I not seen this cited anywhere before? And two, how ironic that he works at Google, and as far as I can tell, the Google resources are one of the ones that don’t specifically cite these baseline requirements.

4. MDN: Progressive Web Apps

Progressive web apps use modern web APIs along with traditional progressive enhancement strategy to create cross-platform web applications. These apps work everywhere and provide several features that give them the same user experience advantages as native apps.

PWAs should be discoverable, installable, linkable, network independent, progressive, re-engageable, responsive, and safe.

Like the Google Developers: Your First Progressive Web App page, this list adds “progressive”, and contains all of the other attributes from the Russell/Berriman list, except for “Fresh” and “App-like”, for some reason?

5. Smashing Magazine: A Beginner’s Guide To Progressive Web Apps

A progressive web application takes advantage of the latest technologies to combine the best of web and mobile apps . Think of it as a website built using web technologies but that acts and feels like an app. Recent advancements in the browser and in the availability of service workers and in the Cache and Push APIs have enabled web developers to allow users to install web apps to their home screen, receive push notifications and even work offline.

It also recites the same characteristics, linking back to the Google Developers resource. It also cites PWAs as being “originally proposed by Google”.

The end of this unnecessary journey

Now, finally, what I will take forward as my personal definition of a PWA:

“Progressive web app” (PWA) is both a general term for a new philosophy toward building websites and a specific term with an established set of three explicit, testable, baseline requirements.

As a general term, the PWA approach is characterized by striving to satisfy the following set of attributes:

  1. Responsive
  2. Connectivity independent
  3. App-like-interactions
  4. Fresh
  5. Safe
  6. Discoverable
  7. Re-engageable
  8. Installable
  9. Linkable

As a specific term, websites can be tested against the following three baseline criteria to qualify as a PWA:

  1. It must run under HTTPS.
  2. It must include a Web App Manifest.
  3. It must implement a service worker.

The general set of attributes was first explicitly enumerated by Alex Russell and Frances Berriman, and the set of three requirements can also be attributed to Alex Russell.

PWAs are apps delivered through the web (as opposed to native apps, packaged and deployed through stores). As Alex Russell said:

…they’re just websites that took all the right vitamins.

Timeline of referenced posts:

Sidenote: I mostly drafted this post on a plane, with no wifi. I realized I had forgotten to re-open one of Jeremy Keith’s posts I meant to reference. I clicked it, and there it was, cached and ready for my offline perusal 🙃

Top comments (16)

chiangs profile image
Stephen Chiang

I love the PWA concept and it's a great way for smaller web apps to grab some of the share for the user's time without having to compete in an app market. Because it is progressive, it is supposed to work no matter what browser is used, but the challenge or frustrating part is that you can't control which browser the user is on so the level and depth of PWA support is not predictable. My personal portfolio is a PWA (just for fun) and doesn't tap into any native mobile features like camera, but the offline access and caching alone is pretty awesome. Nice article and well written!

ccarvalho profile image
Carlos Eduardo

I could't fail to notice that one of the links quoted will be written in the future!! Okay, not exactly. The article "Yes, That Web Project Should Be a PWA" wasn't written in 2018, it was written in 2017. There is a quote in the body of your aticle and in the timeline.

Beside that, great great article.

amberleyjohanna profile image
Amberley Romo

Nice catch -- updated!

tunaxor profile image
Angel Daniel Munoz Gonzalez

Great article!

I would just add that if you are into Windows Development, PWA's Have access to WinRT API (all native stuff from windows devices) if you publish them as an application in the windows store!

you only need to create a project in VS2017 select the PWA template, open the manifest file, point your App to your already published PWA and that's it.

while not many websites need WinRT API access, I think it should be possible for games built with something like Phaser 3 to use the PWA off-line capabilities or even push notifications.

If you are into the windows stuff perhaps access to the windows store either for exposure or as a mechanism to distribute content for micro-transsactions

owlypixel profile image

It seems like Microsoft wants to make PWAs first-class citizens on Windows. As they announced recently, PWAs there have really tasty features like:

  • The ability to set the dark and light mode. Having access to the API to know whether the color is dark or light gives your app the ability to sort-of blend in🕵️.
  • Web standard for sharing. Your web app can be shared to target, which is actually a big deal.
  • System media controls. You can control certain aspects of your app like volume from the keyboard, just like a native app.
  • Hardware and file system access is a really significant feature too.

Besides, the Dev Center gives you some pretty cool analytics about your web app and integration with the Feedback Hub is nice too.

All this really makes me want to try it out sometime.

tunaxor profile image
Angel Daniel Munoz Gonzalez

Yeah! there are a couple UWP apps that I have wrote about here on that I'd like to port into a PWA for the Windows Store, I have a proof of concept for a grocery todo list for my wife and I, but I'd like to do something more insightful like the system media controls and file system access

developius profile image
Finnian Anderson
nickytonline profile image
Nick Taylor

Great writeup Amberley. I must admit, that aside from my blog, which uses Gatsby and comes kinda PWA out of the box, I have not really had a chance still to build a PWA. Gonna Bookmark this one. 😉

amberleyjohanna profile image
Amberley Romo

Thanks Nick! With Gatsby it does come out of the box (sort of). Did you include the gatsby-plugin-offline and gatsby-plugin-manifest? Just updated the PWA page on the v2 docs

nickytonline profile image
Nick Taylor

I went with a template a while back that uses an older version of Gatsby, but I do have the offline plugin installed, No manifest plugin though. Is that a newer plugin or I guess I'm just missing it? 🙃

I need to upgrade my blog to the latest and greatest as there have been many improvements since the version I'm using.

nickytonline profile image
Nick Taylor

Upgraded my starter theme for gatsby and added the manifest plugin (already had offline). Works great!

andrewdtanner profile image
Andrew Tanner 🇪🇺

Thanks for the summary. I've been meaning to get my hands dirty with PWA's (as well as everything else) so this is a great reference for when I get around to it.

jdsteinhauser profile image
Jason Steinhauser

I ❤ consolidated history posts like this. Fantastic work, and this is a resource I will keep bookmarked!

dandevri profile image
Danny de Vries

Really liked this interview with Matt Gaunt on 'Designer vs Developer' where they talk about some difference between the web, PWA's and native.

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