Progress Delayed Is Progress Denied

Three facts...

  1. Apple forces developers of competing browsers to use their engine for all browsers on iOS, restricting their ability to deliver a better version of the web platform.
  2. Apple claims that browsers on iOS are platforms sufficient to support developers who object to the App Store's terms .

...and a proposition:

Apple's iOS browser (Safari) and engine (WebKit) are uniquely under-powered. Consistent delays in the delivery of important features ensure the web can never be a credible alternative to its proprietary tools and App Store.

This is a bold assertion, and proving it requires overwhelming evidence. This post mines publicly available data on the pace of compatibility fixes and feature additions to assess the claim.

Steve & Tim's Close-up Magic #

Misdirections often derail the debate around browsers, the role of the web, and App Store policies on iOS. Classics of the genre include:

Apple's just focused on performance!
...that feature is in Tech Preview
Apple's trying, they just added <long-awaited feature>

These points can be simultaneously valid and immaterial to the web's fitness as a competent alternative to native app development on iOS.

To understand gaps in capabilities Apple has created and maintains between the web and native, we should look at trends rather than individual releases. To know if we're in a drought, we have to look at reservoir water levels and seasonal rainfall. It might be raining features right this instant, but weather isn't climate.

Before we get to measuring water levels, I want to make some things excruciatingly clear.

First, what follows is not a critique of individuals on the Safari team or the WebKit project; it is a plea for Apple to fund their work adequately[2]. They are, pound for pound, some of the best engine developers globally and genuinely want good things for the web. Apple Corporate is at fault, not Open Source engineers or the line managers who support them.

Second, browser projects having different priorities at the leading edge is natural and healthy. So is speedy resolution and agreement. What's unhealthy is an engine trailing far behind for many years. Even worse are situations that cannot be addressed through browser choice. It's good for teams to be leading in different areas, assuming that the "compatible core" of features continues to expand at a steady pace. We should not expect uniformity in the short run — it would leave no room for leadership[3].

Lastly, while this post does measure the distance Safari lags, let nobody mistake that for the core concern: iOS App Store policies that prevent meaningful browser competition are at issue here.

Safari trails competing MacOS browsers by roughly the same amount, but it's not a crisis for the web because genuine browser choice provides users and developers meaningful alternatives. MacOS Safari is compelling enough to have held a 40-50% share for many years running, and do so amidst stiff competition. Safari has many good features, and in an open marketplace, choosing it (or not) is entirely reasonable. That is not the topic of this post.

The Performance Argument #

As an engineer on a browser team, I'm privy to the blow-by-blow of various performance projects, benchmark fire drills, and the ways performance marketing (deeply) impacts engineering priorities.

All modern browsers are fast, Chromium and Safari/WebKit included. No browser is always fastest in every area today. As reliably as the Sun rises in the East, benchmarks lost launch projects to re-architect internals, intending to pull ahead. This is as it should be.

Healthy competitions feature competitors trading the lead with regularity. Spurious reports of "10x worse" performance merit intense scepticism.

After 20 years of neck-in-neck competition, often starting from common code lineages, there just isn't that much left to wring out of the system. Consistent improvement is the name of the game, and it can still have positive impacts, particularly as users lean on the system more heavily over time.

All browsers are deep into the optimisation journey, forcing complex tradeoffs. Improving things for one type of device or application can regress them for others. Significant gains today tend to come from (subtly) breaking contracts with developers in the hopes users won't notice. There isn't a massive gap in focus on performance engineering between engines.

Small gaps and a frequent hand-off of the lead imply differences in capability and correctness aren't the result of one team focusing on performance while others chase different goals[4].

Finally, the choice to fund feature and correctness work is not mutually exclusive to improving performance. Many delayed features on the list below would allow web apps to run faster on iOS. Internal re-architectures to improve correctness often yield performance benefits too.

The Compatibility Tax #

Web developers are a hearty bunch; we don't give up at the first whiff of bugs or incompatibility between engines. Deep wells of knowledge and practice centre on the question: "how can we deliver a good experience to everyone despite differences in what their browsers support?"

Adaptation is a way of life for skilled front enders.

The cultural value of adaptation has enormous implications. First, web developers don't view a single browser as their development target. Education, tools, and training all support the premise that supporting more browsers is better (ceteris paribus), creating a substantial incentive to grease squeaky wheels. Therefore, bridging the gap between leading and trailing-edge browsers is an intense focus of the web development community. Huge amounts of time and effort are spent developing workarounds (preferably with low runtime cost) for lagging engines[5]. Where workarounds fail, cutting features and UI fidelity is understood to be the right thing to do.

Put another way; it's possible to deny web developers access to features simply by failing to deliver them at the margin. For web developers, compatibility across engines is key to productivity. To the extent that an engine has more than 10% share (or thereabouts), developers tend to view features it lacks as "not ready".

To judge the impact of iOS along this dimension, we can try to answer a few questions:

  1. How far behind both competing engines is Safari regarding correctness?
  2. When Safari has implemented essential features, how often is it far ahead? Behind?

Thanks to the Web Platform Tests project and, we have the makings of an answer for the first:

The yellow Safari line is a rough measure of how often other browsers are compatible, but Safari's implementation is wrong. Conversely, the much lower Chrome and Firefox lines indicate Blink and Gecko are considerably more likely to agree and be correct regarding core web standards[6].

In almost every area, Apple's low-quality implementation of features WebKit already supports requires workarounds. Developers would not need to find and fix these issues in Firefox (Gecko) or Chrome/Edge/Brave/Samsung Internet (Blink). This adds to the expense of developing for iOS.

Converging Views #

The Web Confluence Metrics project provides another window into this question.

This dataset is derived by walking the tree of web platform features exposed to JavaScript, an important subset of features. The available data goes back further, providing a fuller picture of the trend lines of engine completeness.

Engines add features at different rates, and the Confluence graphs illuminate both the absolute scale of differences and the pace at which releases add new features. The data is challenging to compare across those graphs, so I extracted it to produce a single chart:


In line with Web Platform Tests data, Chromium and Firefox implement more features and deliver them to market more steadily. From this data, we see that iOS is the least complete and competitive implementation of the web platform, and the gap is growing. At the time of the last Confluence run, the gap had stretched to nearly 1000 APIs, doubling since 2016.

Perhaps counting APIs gives a distorted view?

Some minor additions (e.g. CSS's new Typed Object Model) may feature large expansions in API surface. Likewise, some transformative APIs (e.g. webcam access via getUserMedia() or Media Sessions) may only add a few methods and properties.

To understand if intuitions formed by the Web Confluence data are directionally correct, we need to look more deeply at the history of feature development and connect APIs to the types of applications they enable.

Material Impacts #

Browser release notes and caniuse tables since Blink forked from WebKit in 2013[7] capture the arrival of features in each engine over an even longer period than either WPT or the Confluence dataset. This record can inform a richer understanding of how critical individual features and sets of capabilities unlock new types of apps on the web.

Browsers sometimes launch new features simultaneously (e.g., CSS Grid and ES6). More often, there is a lag between the first and the rest. To provide a sizeable "grace period", and account for short-run differences in engine priorities, we look primarily at features with a gap of three years or more[8].

What follows is an attempt at a full accounting of features launched in this era. A summary of each API and the impact of its absence accompanies every item.

Where Chrome Has Lagged #

It's healthy for engines to have different priorities, leading every browser to avoid certain features. Chrome has missed several APIs for 3+ years:

Where iOS Has Lagged #

Some features in this list were launched in Safari but were not enabled for other browsers forced to use WebKit on iOS (e.g. Service Workers, getUserMedia). In these cases, only the delay to shipping in Safari is considered.

Not every feature blocked or delayed on iOS is transformative, and this list omits cases that were on the bubble (e.g., the 2.5 year lag for BigInt). Taken together, the delays Apple generates, even for low-controversy APIs, makes it challenging for businesses to treat the web as a serious development platform.

The Price #

Suppose Apple had implemented WebRTC and the Gamepad API in a timely way. Who can say if the game streaming revolution now taking place might have happened sooner? It's possible that Amazon Luna, NVIDIA GeForce NOW, Google Stadia, and Microsoft xCloud could have been built years earlier.

It's also possible that APIs delivered on every other platform, but not yet available on any iOS browser (because Apple), may hold the key to unlocking whole categories of experiences on the web.

While dozens of features are either currently, or predicted to be, delayed multiple years by Apple, a few high-impact capabilities deserve particular mention:

These omissions mean web developers cannot compete with their native app counterparts on iOS in critical categories like gaming, shopping, and creative tools.

Developers expect some lag between the introduction of native features and corresponding browser APIs. Apple's policy against browser engine choice adds years of delays beyond the (expected) delay of design iteration, specification authoring, and browser feature development.

These delays prevent developers from reaching wealthy users with great experiences on the web. This gap, created exclusively and uniquely by Apple policy, all but forces businesses off the web and into the App Store where Apple prevents developers from reaching users with web experiences.

Just Out Of Reach #

One might imagine five-year delays for 3D, media, and games might be the worst impact of Apple's policies preventing browser engine progress. That would be mistaken.

The next tier of missing features is relatively uncontroversial proposals in development in standards groups that Apple participates in or has enough support from web developers to be "no-brainers". Each enables better quality web apps. None are expected on iOS any time soon:

Fewer of these features are foundational (e.g. SIMD). However, even those that can be emulated in other ways still impose costs on developers and iOS users to paper over the gaps in Apple's implementation of the web platform. This tax can, without great care, slow experiences for users on other platforms as well[12].

What Could Be #

Beyond these relatively uncontroversial (MIA) features lies an ocean of foreclosed possibility. Were Apple willing to allow the sort of honest browser competition for iOS that MacOS users enjoy, features like these would enable entirely new classes of web applications. Perhaps that's the problem.

Some crucial features (shipped on every other OS) that Apple is preventing any browser from delivering to iOS today, in no particular order:

The list of missing, foundational APIs for media, social, e-commerce, 3d apps, and games is astonishing. Essential apps in the most popular categories in the App Store are impossible to attempt on the web on iOS because of feature gaps Apple has created and perpetuates.

Device APIs: The Final Frontier #

An area where browsers makers disagree fervently, but where Chromium-based browsers have forged ahead (Chrome, Edge, Samsung Internet, Opera, UC, etc.) is access to hardware devices. While not essential to most "traditional" web apps, these features are foundational for vibrant categories like education and creative music applications. iOS Safari supports none of them today, while Chromium browsers on other OSes enable these apps on the web:

Each entry in this inexhaustive list can block entire classes of applications from credibly being possible on the web. The real-world impact is challenging to measure. Weighing up the deadweight losses seems a good angle for economists to investigate. Start-ups not attempted, services not built, and higher prices for businesses forced to develop native apps multiple times could, perhaps, be estimated.

Incongruous #

The data agree: Apple's web engine consistently trails others in both compatibility and features, resulting in a large and persistent gap with Apple's native platform.

Apple wishes us to accept that:

  • It is reasonable to force iOS browsers to use its web engine, leaving iOS on the trailing edge.
  • The web is a viable alternative on iOS for developers unhappy with App Store policies.

One or the other might be reasonable. Together? Hmm.

Parties interested in the health of the digital ecosystem should look past Apple's claims and focus on the differential pace of progress.

Full disclosure: for the past twelve years I have worked on Chromium at Google, spanning both the pre-fork era where potential features for Chrome and Safari were discussed within the WebKit project, as well as the post-fork epoch. Over this time I have led multiple projects to add features to the web, some of which have been opposed by Safari engineers.

Today, I lead Project Fugu, a collaboration within Chromium that is directly responsible for the majority of the device APIs mentioned above. Microsoft, Intel, Google, Samsung, and others are contributing to this work, and it is being done in the open with the hope of standardisation, but my interest in its success is large. My front-row seat allows me to state unequivocally that independent software developers are clamouring for these APIs and are ignored when they request support for them from Apple. It is personally frustrating to be unable to deliver these improvements for developers who wish to reach iOS users — which is all developers. My interests and biases are plain.

Previously, I helped lead the effort to develop Service Workers, Push Notifications, and PWAs over the frequent and pointed objections of Apple's engineers and managers. Service Worker design was started as a collaboration between Google, Mozilla, Samsung, Facebook, Microsoft, and independent developers looking to make better, more reliable web applications. Apple only joined the group after other web engines had delivered working implementations. The delay in availability of Service Workers (as well as critical follow-on features like Navigation Preload) for iOS users and developers interested in serving them well, likewise, carries an undeniable personal burden of memory.