Infrequently Noted

Alex Russell on browsers, standards, and the process of progress.

A Funny Thing Happened On The Way To The Future...

There's a post on the fetch() API by Ludovico Fischer doing the rounds. As a co-instigator for adding the API to the platform, it's always a curious thing to read commentary about an API you designed, but this one more than most. It brings together the epic slog that was the Promises design (which we also waded into in order to get Service Workers done and which will improve with await/async) with the in-process improvements that will come from Streams and it mixes it with a dollop of FUD, misunderstanding, and derision.

This sort of article is emblematic of a common misunderstanding. It expresses (at the end) a latent belief that there is some better alternative available for making progress on the web platform than to work feature-by-feature, compromise-by-compromise towards a better world. That because the first version of fetch() didn't have everything we want, it won't ever. That there was either some other way to get fetch() shipped or that there was a way to get cancellation through TC39 in '13. Or that subclassing is somehow illegitimate and "non-standard" (even though the subclass would clearly be part of the Fetch Standard).

These sorts of undirected, context-free critiques rely on snapshots of the current situation (particularly deficiencies thereof) to argue implicitly that someone must be to blame for the current situation not yet being as good as the future we imagine. To get there, one must apply skepticism about all future progress; "who knows when that'll be done!" or "yeah, fine, you shipped it but it isn't available in Browser X!!!".

They're hard to refute because they're both true and wrong. It's the prepper/end-of-the-world mentality applied to technological progress. Sure, the world could come to a grinding halt, society could disintegrate, and the things we're currently working on could never materialize. But, realistically, is that likely? The worst-case-scenario peddlers don't need to bother with that question. It's cheap and easy to "teach the controversy". The appearance of drama is its own reward.

Perhaps most infuriatingly, these sorts of cheap snapshots laced with FUD do real harm to the process of progress. They make it harder for the better things to actually appear because "controversy" can frequently become a self fulfilling prophesy; browser makers get cold feet for stupid reasons which can create negative feedback loops of indecision and foot-gazing. It won't prevent progress forever, but it sure can slow it down.

I'm disappointed in SitePoint for publishing the last few paragraphs in an otherwise brilliant article, but the good news is that it (probably) won't slow Cancellation or Streams down. They are composable additions to fetch() and Promises. We didn't block the initial versions on them because they are straightforward to add later and getting the first versions done required cuts. Both APIs were designed with extensions in mind, and the controversies are small. Being tactical is how we make progress happen, even if it isn't all at once. Those of us engaged in this struggle for progress are going to keep at it, one feature (and compromise) at a time.

Progressive Web Apps: Escaping Tabs Without Losing Our Soul

It happens on the web from time to time that powerful technologies come to exist without the benefit of marketing departments or slick packaging. They linger and grow at the peripheries, becoming old-hat to a tiny group while remaining nearly invisible to everyone else. Until someone names them.

This may be the inevitable consequence of a standards-based process and unsynchronized browser releases. We couldn't keep a new feature secret if we wanted to, but that doesn't mean anyone will hear about it. XMLHTTPRequest was available broadly since IE 5 and in Gecko-based browsers from as early as 2000. "AJAX" happened 5 years later.

This eventual adding-up of new technologies changes how we build and deliver experiences. They succeed when bringing new capabilities while maintaining shared principles:

Major evolutions of the web must be compatible with it culturally as well as technically.

Many platforms have attempted to make it possible to gain access to "exotic" capabilities while still allowing developers to build with the client-side technology of the web. In doing so they usually jettison one or more aspect of the shared value system. They aren't bad — many are technically brilliant — but they aren't of the web:

These are just the ones that spring to mind offhand. I'm sure there have been others; it's a popular idea. They frequently give up linkability in return for "appiness": to work offline, be on the home screen, access system APIs, and re-engage users they have required apps be packaged, distributed through stores, and downloaded entirely before being experienced.

Instead of clicking a link to access the content you're looking for, these systems make stores the mediators of applications which in turn mediate and facilitate discovery for content. The hybridzation process generates applications which can no longer live in or with the assumptions of the web. How does one deploy to all of these stores all at once? Can one still keep a fast iteration pace? How does the need to package everything up-front change your assumptions and infrastructure? How does search indexing work? It's a deep tradeoff that pits fast-iteration and linkability against offline and store discovery.

Escaping the Tab: Progressive, Not Hybrid

But there is now another way. An evolution has taken place in browsers.

Over dinner last night, Frances and I enumerated the attributes of this new class of applications:

These apps aren't packaged and deployed through stores, they're just websites that took all the right vitamins. They keep the web's ask-when-you-need-it permission model and add in new capabilities like being top-level in your task switcher, on your home screen, and in your notification tray. Users don't have to make a heavyweight choice up-front and don't implicitly sign up for something dangerous just by clicking on a link. Sites that want to send you notifications or be on your home screen have to earn that right over time as you use them more and more. They progressively become "apps".

Critically, these apps can deliver an even better user experience than traditional web apps. Because it's also possible to build this performance in as progressive enhancement, the tangible improvements make it worth building this way regardless of "appy" intent.

Frances called them "Progressive Open Web Apps" and we both came around to just "Progressive Apps". They existed before, but now they have a name.

What Progressive Apps Look Like

Taking last year's Chrome Dev Summit site as an example, we can see the whole flow in action (ht: Paul Kinlan):

  1. The site begins life as a regular tab. It doesn't have super-powers, but it is built using Progressive App features including TLS, Service Workers, Manifests, and Responsive Design.
  2. The second (or third or fourth) time one visits the site -- roughly at the point where the browser it sure it's something you use frequently -- a prompt is shown by the browser (populated from the Manifest details)
  3. Users can decide to keep apps to the home screen or app launcher
  4. When launched from the home screen, these apps blend into their environment; they're top-level, full-screen, and work offline. Of course, they worked offline after step 1, but now the implicit contract of "appyness" makes that clear.
Progressive Web App installation flow for Chrome Dev Summit.
Progressive Web App installation flow for Chrome Dev Summit.

Here's the same flow on Flipboard today:

Progressive Apps are web apps, they begin life in a tab. Here we see in Chrome for Android with regular tab treatment.
Progressive Apps are web apps, they begin life in a tab. Here we see in Chrome for Android with regular tab treatment.
When users engage with Progressive Web Apps enough, browsers offer prompts that ask users if they want to keep them. To avoid spaminess, this doesn't happen on the first load.
When users engage with Progressive Web Apps enough, browsers offer prompts that ask users if they want to keep them. To avoid spaminess, this doesn't happen on the first load.
If the user accepts, the user's flow isn't interrupted.
If the user accepts, the user's flow isn't interrupted.
The app shortcut appears on the homescreen or launcher of the OS.
The app shortcut appears on the homescreen or launcher of the OS.
When launched, Progressive Web Apps can choose to be full-screen.
When launched, Progressive Web Apps can choose to be full-screen.
Progressive Web Apps are top-level activities in the OS's application switcher.
Progressive Web Apps are top-level activities in the OS's application switcher.

The Future

Today's web development tools and practices don't yet naturally support Progressive Apps, although many frameworks and services are close enough to be usable for making Progressive Apps. In particular, client-side frameworks that have server-rendering as an option work well with the model of second-load client-side routing that Progressive Apps naturally adopt as a consequence of implementing robust offline experiences.

This is an area where thoughtful application design and construction will give early movers a major advantage. Full Progressive App support will distinguish engaging, immersive experiences on the web from the "legacy web". Progressive App design offers us a way to build better experiences across devices and contexts within a single codebase but it's going to require a deep shift in our understanding and tools.

Building immersive apps using web technology no longer requires giving up the web itself. Progressive Apps are our ticket out of the tab, if only we reach for it.

Thanks to Frances Berriman, Brian Kardell, Jake Archibald, Owen Cambpell-Moore, Jan Lehnardt, Mike Tsao, Yehuda Katz, Paul Irish, Matt McNulty, and John Allsopp for their review and suggestions on this post.

Cross-posted at Medium.

PSA: Service Workers are Coming


This post describes the potential amplification of existing risks that Service Workers bring for multi-user origins where the origin may not fully trust the content or, in which, users should not be able to modify each other's content.

Sites hosting multiple-user content in separate directories, e.g. /~alice/index.html and /~bob/index.html, are not exposed to new risks by Service Workers. See below for details.

Sites which host content from many users on the same origin at the same level of path separation (e.g. and may need to take precaution to disable Service Workers. These sites already rely on extraordinary cooperation between actors and are likely to find their security assumptions astonished by future changes to browsers.


Service Workers are a new feature that are coming to the Web Platform very soon.

Like AppCache, Service Workers are available without user prompts and enable developers to create meaningful offline experiences for web sites. They are, however, strictly more powerful than AppCache.

To mitigate the risks associated with request interception, Service Workers are only available to use under the following restrictions:

In addition to these restrictions, Service Workers include features to help site operators understand Service Worker usage on their origins. The most important of these is the Service-Worker: script header which is appended to every request for script files which are intended for use as Service Workers.

This feature allows site owners, via logs and server-side directives, to:

Disabling Service Workers is straightforward. Here's an example snippet for an Apache .htaccess file:

<IfModule mod_setenvif.c>
  SetEnvIf Service-Worker script swrequest
    Require all granted
    Require not env swrequest

For Nginx the recipe might be:

location / {
  if ($http_service_worker) {
    return 403;


If you run a site which hosts untrusted third-party content on a single origin over SSL/TLS, you should ensure that you:

Thanks to Kenji Baheux, Joel Weinberger, Devdatta Akhawe, and Matt Falkenhagen for their review and suggestions. All errors are mine alone, however.

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