Using grid lines is a flexible and powerful way to place your elements on a grid, but, when looking at the code, it might be a bit hard to visualize. So Grid gives you another way to place your elements that might be easier to see called grid areas. There’s a bit more upfront work involved, but the pay off might be worth it.

The idea behind grid areas is that we name each element we want to place on our grid and then use a certain syntax to place them visually where we want them to go. To illustrate, let’s start with my product page.

Here’s the html I’m starting with:

<div class="card">
  <h1>Product 1</h1>
  <h2>$99</h2>
  <p>This is a description of the first option.</p>
  <ul><!-- product details here --></ul>
</div>

<div class="card">
  <h1>Product 2</h1>
  <h2>$99</h2>
  <p>This is a description of the first option.</p>
  <ul><!-- product details here --></ul>
</div>

<div class="card">
  <h1>Add-ons</h1>
  <h2>$149</h2>
  <p>This is another description.</p>
  <ul><!-- product details here --></ul>
</div>

<div class="card">
  <h1>Testimonial</h1>
  <h2>$299</h2>
  <p>This is a third description.</p>
  <ul><!-- product details here --></ul>
</div>

And here’s the CSS I’m starting with:

.pricing-options {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 2em;
}
.card {
  border: 1px solid black;
  padding: 2em 5em;
  font-family: sans-serif;
  border-radius: 1em;
}

That will give me this result:

We want our page to follow this design:

This means we want our Add-ons to span two rows and our Testimonial to span two columns. Here’s how we can accomplish that with grid-template-areas.

First, we need to label each element by assigning a value to the grid-area property. To do that, let’s give our elements classes to reference in our CSS.

<div class="card product-1">
  <h1>Product 1</h1>
  <!-- product details here -->
</div>

<div class="card product-2">
  <h1>Product 2</h1>
  <!-- product details here -->
</div>

<div class="card add-ons">
  <h1>Add-ons</h1>
  <!-- product details here -->
</div>

<div class="card testimonial">
  <h1>Testimonial</h1>
  <!-- product details here -->
</div>

Now let’s add those grid-area property values. You can name them anything you want, but since my class names are pretty descriptive, I’m going to name the grid-areas the same as my class names.

.product-1 {
  grid-area: product-1;
}
.product-2 {
  grid-area: product-2;
}
.add-ons {
  grid-area: add-ons;
}
.testimonial {
  grid-area: testimonial;
}

Now comes the visual part. In our CSS, we’re going to go back up to where our grid is defined and we’re going to assign a new property called grid-template-areas. I’m now going to visually assign each cell of my grid to an element in my html.

My grid is three columns across and an unspecified number of rows, which means it’ll be as many rows as needed. I’m going to assign product-1 to that first top cell and product-2 in my second top cell. Then my third top cell is going to be taken up by my add-ons element. And because my add-ons element is going to span two rows, the cell right beneath that is going to be assigned to add-ons too. And finally, we have the bottom left cell assigned to testimonial and because that element is taking up two columns across, the cell right next to it will be assigned to testimonial as well.

Here’s what the final code looks like:

.pricing-options {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 2em;
  grid-template-areas:
    "product-1 product-2 add-ons"
    "testimonial testimonial add-ons";
}

The beauty of grid-template-areas is that all of the decisions about where to place what element happen in a single property. You still have to do the upfront work of naming your elements, but once you’ve done that, you can visually see where everything is in relation to each other in a single place. Changing it is simpler too — just move the element name to a different “cell” and you’re done.

Using grid-template-areas , we get our desired layout.

If you have any other questions about Grid and would like to see more content on this topic, let us know. You can share your feedback with me, Saron Yitbarek, on BlueSky, or reach out to our other evangelists — Jon Davis, on Bluesky / Mastodon, and Jen Simmons, on Bluesky / Mastodon. You can also follow WebKit on LinkedIn. If you find a bug or problem, please file a WebKit bug report.

Update on what happened in WebKit in the week from November 10 to November 17.

This week's update is composed of a new CStringView internal API, more MathML progress with the implementation of the "scriptlevel" attribute, the removal of the Flatpak-based SDK, and the maintanance update of WPEBackend-fdo.

Cross-Port 🐱

Releases 📦️

Infrastructure 🏗️

Safari Technology Preview Release 232 is now available for download for macOS Tahoe and macOS Sequoia. If you already have Safari Technology Preview installed, you can update it in System Settings under General → Software Update.

This release includes WebKit changes between: 301766@main…302449@main.

CSS

New Features

• Added support for allowing positioned boxes in scrollable containing blocks to overflow along scrollable directions. (302259@main) (162722820)
• Added support for flip-x and flip-y options in position-try-fallback for CSS Anchor Positioning. (302057@main) (163282036)

Resolved Issues

• Fixed handling of padding and margins for flex and grid layouts across all writing modes. (301814@main) (71046552)
• Fixed getComputedStyle("top") to correctly resolve percentage values for absolutely positioned elements inside inline containers. (302090@main) (161390162)
• Fixed an infinite style resolution loop when a position-try box was inside a display: none subtree. (302254@main) (161570947)
• Fixed width, height, min-width, min-height, max-width and max-height to apply CSS zoom at used-value time. (302241@main) (161848512)
• Fixed CSS zoom to scale <iframe> element contents. (302097@main) (162314059)
• Fixed getBoundingClientRect and getClientRects to return scaled lengths according to CSS zoom instead of unscaled values, aligning with the CSS Viewport specification. (301806@main). (162325730)
• Fixed scrolling behavior so that scrollRectToVisible() can bring fixed anchor-positioned boxes outside the viewport into view, improving keyboard navigation. (302368@main) (162378346)
• Fixed an issue where @font-face and FontFace.family failed when the font family name contained spaces, ensuring the family name is now treated as a plain string instead of being parsed. (301793@main) (162637501)
• Fixed top, left, right, and bottom to apply CSS zoom at used-value time (302102@main) (162663056)
• Fixed margin to apply CSS zoom at used-value time. (301965@main) (162907254)
• Fixed evaluation of calc() expressions to correctly apply the used zoom factor to length values, ensuring properties like line-height and box dimensions scale properly. (301968@main) (163141549)
• Fixed an issue where calc(em) values for unzoomed properties were incorrectly adjusted. (302041@main) (163267333)
• Fixed position-area normal alignment to correctly align toward the non-auto inset when only one inset is auto. (302299@main) (163317238)
• Fixed incorrect underline positioning for text-decoration when inline box sides are trimmed. (302435@main) (163858721)

JavaScript

Resolved Issues

• Fixed Intl.DateTimeFormat to throw a RangeError for legacy non-IANA timezones, aligning behavior with TC39 standards. (302447@main) (156857252)

Media

Resolved Issues

• Fixed: Aligned with other browsers by dispatching enter and exit events on TextTrackCue and VTTCue with no track. (301836@main) (160195643)
• Fixed an issue where the mute button disappeared in macOS inline videos with adjustable sizes. (301896@main) (162897286)

Rendering

Resolved Issues

• Fixed over-aggressive clipping of child layers in multicolumn layouts to prevent visual overflow issues with position: relative elements and transform:scale() text. (302249@main) (126413036)
• Fixed unreadable Scroll-to-Text-Fragment highlights on dark pages. (301930@main) (126539910)
• Fixed an issue where positioned, transformed, or opacity-altered <img> elements with HDR JPEG gainmaps would incorrectly render in SDR. (302200@main) (156858374)

SVG

Resolved Issues

• Fixed animation of the stop-color attribute on <stop> elements.(302163@main) (109823555)

Storage

Resolved Issues

• Fixed an issue where IndexedDB databases might have mismatched metadata version and database name encoding format. (302055@main) (163219457)

Web API

Resolved Issues

• Fixed event ordering and committed promise timing for intercepted Navigation API traverse navigations. (302418@main) (161445256)
• Fixed the processing order of Trusted Types for DOM attribute setting. (302272@main) (162143148)
• Fixed NavigateEvent to correctly fire an AbortSignal when a navigation is aborted. (302591@main) (163957784)
• Fixed NavigateEvent.sourceElement to correctly reference elements from different browsing contexts. (302504@main) (163962362)

Web Inspector

Resolved Issues

• Fixed an issue where CSS properties added to new rules were not applied and were marked as invalid. (302301@main) (103548968)
• Fixed an issue in the Console where the count of identical consecutive messages could be wrong. (301917@main) (162612099)

Update on what happened in WebKit in the week from November 3 to November 10.

This week brought a hodgepodge of fixes in Temporal and multimedia, a small addition to the public API in preparation for future work, plus advances in WebExtensions, WebXR, and Android support.

Cross-Port 🐱

Multimedia 🎥

JavaScriptCore 🐟

WPE WebKit 📟

WPE Android ↗ 🤖

Community & Events 🤝

Update on what happened in WebKit in the week from October 27 to November 3.

A calmer week this time! This week we have the GTK and WPE ports implementing the RunLoopObserver infrastructure, which enables more sophisticated scheduling in WebKit Linux ports, as well as more information in webkit://gpu. On the Trusted Types front, the timing of check was changed to align with spec changes.

Cross-Port 🐱

Graphics 🖼️

Today, Safari 26.1 is available with iOS 26.1, iPadOS 26.1, macOS Sequoia 26.1 and visionOS 26.1, as well as for macOS Sequoia and macOS Sonoma. It contains 2 new features and 36 improvements to existing features.

Relative units in SVG

As part of our most recent efforts on quality, WebKit for Safari 26.1 refactors the way the rendering engine handles CSS Units. This results in giving the SVG engine easier access to relative units, adding support for certain units inside SVG files for the first time. These units include rem; viewport units: vh, vw, vmin, vmax; typography-relative units: rlh, ic, cap, and container query units: cqw, cqi, cqmin, cqmax.

Anchor Positioning improvements

Safari 26.1 includes a dozen improvements to CSS Anchor Positioning. Now WebKit remembers the last successful position-try fallback in CSS anchor positioning to reduce layout jumps when styles change. See below for the full list of changes to Anchor Positioning.

Bug fixes and more

Along with the new features, WebKit for Safari 26.1 includes improvements to existing features.

Accessibility

• Fixed hit testing for scrolled iframe content by adjusting for the frame’s scroll position, ensuring accurate element detection across assistive technologies. (158233884)
• Fixed an issue where VoiceOver reports the wrong radio count with a dynamically inserted radio option. (159937173)
• Fixed exposing content within dynamically expanded details elements in the accessibility tree. (159937257)
• Fixed the target of aria-labelledby not updating its accessibility label after dynamic innerHTML change in the label. (160691619)

CSS Anchor Positioning

• Fixed anchor positioning to handle fragmented multicolumn flows. (156958568)
• Fixed anchor positioning fallbacks to respond to scrolling. (158451016)
• Fixed an issue where container queries doesn’t work with position-try element. (158880410)
• Fixed anchor positioning to account for a left-hand scrollbar in right-to-left and vertical-rl containing blocks. (160723993)
• Fixed handling inline containing blocks for CSS Anchor Positioning. (160892829)
• Fixed an issue where anchor-positioned elements failed to update their position when the default anchor changed. (160892850)
• Fixed an issue where transitioning an element to display: none with transition-behavior: allow-discrete and CSS Anchor Positioning would repeatedly restart the transition. (161421046)
• Fixed position-area for the initial containing block to include the in-flow scrollable area, improving alignment for typical overflow cases. (161997622)
• Fixed position-visibility: anchors-visible visibility heuristic when anchor is clipped by an ancestor container. (160060564)

CSS

• Fixed @media print styles to work when used inside a nested rule. (158608834)
• Fixed: Improved the performance of :has(> .changed) .subject selectors. (159257003)
• Fixed pseudo-class invalidation performance by creating separate RuleSets for attribute selectors like :has([attr=value]) to avoid using universal invalidation. (159257022)
• Fixed an issue where changing the ruby-overhang property did not trigger a layout update, ensuring proper rendering when overhang values change. (159573050)
• Fixed offsetParent to return the fixed-position element’s containing block when it is not the viewport, such as a transformed element. (160892685)
• Fixed <select> fallback styling by removing the outdated background and updating the dropdown arrow. (161104364)

Forms

• Fixed native text inputs so that their background colors update when autofilled. (159014135)
• Fixed checkboxes and radio buttons missing borders in the filled state when “Increased Contrast” is enabled on macOS. (159379948)

PDF

• Fixed VoiceOver not recognizing the password form in encrypted documents. (159240531)

Rendering

• Fixed a bottom gap appearing on layouts with viewport-sized fixed containers on iOS. (158055568)
• Fixed an issue on iOS where Safari extension popups and some websites could scroll or flicker unexpectedly. (160216319)
• Fixed list markers overlapping text in certain situations. (160892820)
• Fixed an issue that caused cropped flexbox elements to render incorrectly. (161218029)
• Fixed string search freezing when subject has large number (>1000). (161421015)

SVG

• Fixed absolutely positioned SVGs so that their size correctly accounts for the padding of the containing block when the SVG root is out-of-flow. (160727702)

Security

• Fixed Safari ignoring the style-src-elem Content Security Policy directive ensuring it is checked before falling back to style-src, in line with CSP3 specifications. (157298407)

Web API

• Fixed an issue on iOS 26 where pressing the B button on a gamepad could make a page appear to lose gamepad focus by bypassing the system’s automatic navigation behavior. (159125095)

Web Inspector

• Fixed issue where searching on certain text fails to find matches. (159897282)

WebGPU

• Fixed an issue where GPUQueue.copyExternalImageToTexture could not handle SVG images. (158442476)
• Fixed an issue where video playback using the WebGPU renderer in WebCodecs could display a black screen. (158442539)
• Fixed an issue where WebGPU video textures failed to load in Three.js panoramas. (159918934)

WebKit API

• Fixed a crash when an app uses WKWebView::loadRequest API on background threads. (162225842)

WebRTC

• Fixed getUserMedia() on iOS incorrectly firing devicechange events when there was no actual change to available microphones or default devices. (157693528)

Feedback

We love hearing from you. To share your thoughts, find our web evangelists online: Jen Simmons on Bluesky / Mastodon, Saron Yitbarek on Bluesky / Mastodon, and Jon Davis on Bluesky / Mastodon. You can follow WebKit on LinkedIn. If you run into any issues, we welcome your feedback on Safari UI (learn more about filing Feedback), or your WebKit bug report about web technologies or Web Inspector. If you run into a website that isn’t working as expected, please file a report at webcompat.com. Filing issues really does make a difference.

You can also find this information in the Safari release notes.

Safari Technology Preview Release 231 is now available for download for macOS Tahoe and macOS Sequoia. If you already have Safari Technology Preview installed, you can update it in System Settings under General → Software Update.

This release includes WebKit changes between: 300987@main…301765@main.

CSS

New Features

• Added support for the safe keyword with anchor-center in CSS Anchor Positioning. (301301@main) (155767796)
• Added support for flip-x and flip-y options in position-try-fallback for CSS Anchor Positioning. (302057@main) (163282036)

Resolved Issues

• Fixed handling of padding and margins for flex and grid layouts across all writing modes. (301814@main) (71046552)
• Fixed position-visibility: no-overflow to respond correctly to scrolling. (301211@main) (162173481)
• Fixed: Renamed position-area keywords from x-self-start, x-self-end, y-self-start, and y-self-end to self-x-start, self-x-end, self-y-start, and self-y-end respectively to align with updated CSSWG specifications.(301226@main) (162214793)
• Fixed <iframe> elements so their content correctly respects the page’s usedZoom(). (302097@main) (162314059)
• Fixed auto margins by converting them to zero when position-area or anchor-center is applied in CSS Anchor Positioning. (301662@main) (162809291)

JavaScript

Resolved Issues

• Fixed TypeError messages when calling class or function constructors without new to include the constructor name. (301023@main) (161152354)

Media

Resolved Issues

• Fixed an issue where custom WebVTT caption text size settings did not propagate to cue child elements by moving the font-size definition into the cue’s shared <style> block. (301681@main) (162547969)

Rendering

New Features

• Added support for text shaping across inline boxes. (301354@main) (162430932)

Resolved Issues

• Fixed an issue where selecting table cells could cause overlapping selections in flex and grid layouts. (294464@main) (160805174)
• Fixed a performance issue on layouts with long pre blocks and word-break: break-all by including whitespace in overflow width calculations. (301657@main) (162695099)
• Fixed Largest Contentful Paint to optimize text paints by performing an early area comparison when an element has only one text box. (301895@main) (163067611)
• Fixed Largest Contentful Paint to skip tracking loadTime for data URI images. (301988@main) (163213487)
• Fixed how Largest Contentful Paint performs area checks for text nodes, optimizing calculations when all rects are collected and ancestor transforms are absent. (302072@main) (163285757)

Web API

Resolved Issues

• Fixed Navigation API WPT tests failing due to a WebDriver error. (161199777)
• Fixed PerformanceEventTiming so that keydown and pointerdown entries no longer wait for their corresponding keyup or pointerup events before assigning a duration, preventing durations from appearing too long. (302107@main) (161911473)
• Fixed attachShadow() to use the global custom element registry by default when customElementRegistry is null, aligning with the WHATWG DOM specification (300996@main). (161949493)
• Fixed navigate() with { history: "replace" } to correctly update the current History item instead of adding a new one during same-document navigations. (302130@main) (163323288)

Web Inspector

Resolved Issues

• Fixed an issue where the Sources tab won’t show contents of a script that contains a for statement with optional chaining in the test condition. (301197@main) (160617913)

WebDriver

Resolved Issues

• Fixed an issue where element references nested inside Array or Object arguments were not properly extracted when executing scripts. (301445@main) (162571946)

Update on what happened in WebKit in the week from October 21 to October 28.

This week has again seen a spike in activity related to WebXR and graphics performance improvements. Additionally, we got in some MathML additions, a fix for hue interpolation, a fix for WebDriver screenshots, development releases, and a blog post about memory profiling.

Cross-Port 🐱

Graphics 🖼️

WPE WebKit 📟

WPE Platform API 🧩

Releases 📦️

Community & Events 🤝

One of the main constraints that embedded platforms impose on the browsers is a very limited memory. Combined with the fact that embedded web applications tend to run actively for days, weeks, or even longer, it’s not hard to imagine how important the proper memory management within the browser engine is in such use cases. In fact, WebKit and WPE in particular receive numerous memory-related fixes and improvements every year. Before making any changes, however, the areas to fix/improve need to be narrowed down first. Like any C++ application, WebKit memory can be profiled using a variety of industry-standard tools. Although such well-known tools are really useful in the majority of use cases, they have their limits that manifest themselves when applied on production-grade embedded systems in conjunction with long-running web applications. In such cases, a very useful tool is a debug-only feature of WebKit itself called malloc heap breakdown, which this article describes.

Industry-standard memory profilers #

When it comes to profiling memory of applications on linux systems, the 2 outstanding tools used usually are Massif (Valgrind) and Heaptrack.

Massif (Valgrind) #

Massif is a heap profiler that comes as part of the Valgrind suite. As its documentation states:

It measures how much heap memory your program uses. This includes both the useful space, and the extra bytes allocated for book-keeping and alignment purposes. It can also measure the size of your program’s stack(s), although it does not do so by default.

Using Massif with WebKit is very straightforward and boils down to a single command:

Malloc=1 valgrind --tool=massif --trace-children=yes WebKitBuild/GTK/Debug/bin/MiniBrowser '<URL>'

• The Malloc=1 environment variable set above is necessary to instruct WebKit to enable debug heaps that use the system malloc allocator.

Given some results are generated, the memory usage over time can be visualized using massif-visualizer utility. An example of such a visualization is presented in the image below:

While Massif has been widely adopted and used for many years now, from the very beginning, it suffered from a few significant downsides.

First of all, the way Massif instruments the profiled application introduces significant overhead that may slow down the application up to 2 orders of magnitude. In some cases, such overhead makes it simply unusable.

The other important problem is that Massif is snapshot-based, and hence, the level of detail is not ideal.

Heaptrack #

Heaptrack is a modern heap profiler developed as part of KDE. The below is its description from the git repository:

Heaptrack traces all memory allocations and annotates these events with stack traces. Dedicated analysis tools then allow you to interpret the heap memory profile to:

• find hotspots that need to be optimized to reduce the memory footprint of your application
• find memory leaks, i.e. locations that allocate memory which is never deallocated
• find allocation hotspots, i.e. code locations that trigger a lot of memory allocation calls
• find temporary allocations, which are allocations that are directly followed by their deallocation

At first glance, Heaptrack resembles Massif. However, a closer look at the architecture and features shows that it’s much more than the latter. While it’s fair to say it’s a bit similar, in fact, it is a significant progression.

Usage of Heaptrack to profile WebKit is also very simple. At the moment of writing, the most suitable way to use it is to attach to a certain running WebKit process using the following command:

heaptrack -p <PID>

while the WebKit needs to be run with system malloc, just like in Massif case:

WEBKIT_DISABLE_SANDBOX_THIS_IS_DANGEROUS=1 Malloc=1 WebKitBuild/GTK/Debug/bin/MiniBrowser '<URL>'

• If profiling of e.g. web content process startup is essential, it’s then recommended also to use WEBKIT2_PAUSE_WEB_PROCESS_ON_LAUNCH=1, which adds 30s delay to the process startup.

When the profiling session is done, the analysis of the recordings is done using:

heaptrack --analyze <RECORDING>

The utility opened with the above, shows various things, such as the memory consumption over time:

flame graphs of memory allocations with respect to certain functions in the code:

etc.

As Heaptrack records every allocation and deallocation, the data it gathers is very precise and full of details, especially when accompanied by stack traces arranged into flame graphs. Also, as Heaptrack does instrumentation differently than e.g. Massif, it’s usually much faster in the sense that it slows down the profiled application only up to 1 order of magnitude.

Shortcomings on embedded systems #

Although the memory profilers such as above are really great for everyday use, their limitations on embedded platforms are:

• they significantly slow down the profiled application — especially on low-end devices,
• they effectively cannot be run for a longer period of time such as days or weeks, due to memory consumption,
• they are not always provided in the images — and hence require additional setup,
• they may not be buildable out of the box on certain architectures — thus requiring extra patching.

While the above limitations are not always a problem, usually at least one of them is. What’s worse, usually at least one of the limitations turns into a blocking problem. For example, if the target device is very short on memory, it may be basically impossible to run anything extra beyond the browser. Another example could be a situation where the application slowdown due to the profiler usage, leads to different application behavior, such as a problem that originally reproduced 100% of the time, does not reproduce anymore etc.

Malloc heap breakdown in WebKit #

Profiling the memory of WebKit while addressing the above problems points towards a solution that does not involve any extra tools, i.e. instrumenting WebKit itself. Normally, adding such an instrumentation to the C++ application means a lot of work. Fortunately, in the case of WebKit, all that work is already done and can be easily enabled by using the Malloc heap breakdown.

In a nutshell, Malloc heap breakdown is a debug-only feature that enables memory allocation tracking within WebKit itself. Since it’s built into WebKit, it’s very lightweight and very easy to build, as it’s just about setting the ENABLE_MALLOC_HEAP_BREAKDOWN build option. Internally, when the feature is enabled, WebKit switches to using debug heaps that use system malloc along with the malloc zone API to mark objects of certain classes as belonging to different heap zones and thus allowing one to track the allocation sizes of such zones.

As the malloc zone API is specific to BSD-like OSes, the actual implementations (and usages) in WebKit have to be considered separately for Apple and non-Apple ports.

Malloc heap breakdown on Apple ports #

Malloc heap breakdown was originally designed only with Apple ports in mind, with the reason being twofold:

1. The malloc zone API is provided virtually by all platforms that Apple ports integrate with.
2. MacOS platforms provide a great utility called footprint that allows one to inspect per-zone memory statistics for a given process.

Given the above, usage of malloc heap breakdown with Apple ports is very smooth and as simple as building WebKit with the ENABLE_MALLOC_HEAP_BREAKDOWN build option and running on macOS while using the footprint utility:

Footprint is a macOS specific tool that allows the developer to check memory usage across regions.

For more details, one should refer to the official documentation page.

Malloc heap breakdown on non-Apple ports #

Since all of the non-Apple WebKit ports are mostly being built and run on non-BSD-like systems, it’s safe to assume the malloc zone API is not offered to such ports by the system itself. Because of the above, for many years, malloc heap breakdown was only available for Apple ports.

Fortunately, with the changes introduced in 2025, such as: 294667@main (+ fix 294848@main), 301702@main, and improvements such as: 294848@main, 299555@main, 301695@main, 301709@main, 301712@main, 301839@main, 301861@main, the malloc heap breakdown integrates also with non-Apple ports and is stable as of main@a235408c2b4eb12216d519e996f70828b9a45e19.

The idea behind the integration for non-Apple ports is to provide a simple WebKit-internal library that provides a fake <malloc/malloc.h> header along with simple implementation that provides malloc_zone_*() function implementations as proxy calls to malloc(), calloc(), realloc() etc. along with a tracking mechanism that keeps references to memory chunks. Such an approach gathers all the information needed to be reported later on.

At the moment of writing, the above allows 2 methods of reporting the memory usage statistics periodically:

• printing to standard output,
• reporting to sysprof as counters.

Periodic reporting to standard output

By default, when WebKit is built with ENABLE_MALLOC_HEAP_BREAKDOWN, the heap breakdown is printed to the standard output every few seconds for each process. That can be tweaked by setting WEBKIT_MALLOC_HEAP_BREAKDOWN_LOG_INTERVAL=<SECONDS> environment variable.

The results have a structure similar to the one below:

402339 MHB: | PID | "Zone name" | #chunks | #bytes | {
402339 "ExecutableMemoryHandle" 2 32
402339 "AssemblerData" 1 192
402339 "VectorBuffer" 37 16184
402339 "StringImpl" 103 5146
402339 "WeakPtrImplBase" 17 272
402339 "HashTable" 37 9408
402339 "Vector" 1 16
402339 "EmbeddedFixedVector" 1 32
402339 "BloomFilter" 2 65536
402339 "CStringBuffer" 3 86
402339 "Default Zone" 0 0
402339 } MHB: grand total bytes allocated: 9690

Given the allocation statistics per-zone, it’s easy to narrow down the unusual usage patterns manually. The example of a successful investigation is presented in the image below:

Moreover, the data presented can be processed either manually or using scripts to create memory usage charts that span as long as the application lifetime so e.g. hours (20+ like below), days, or even longer:

Periodic reporting to sysprof

The other reporting mechanism currently supported is reporting periodically to sysprof as counters. In short, sysprof is a modern system-wide profiling tool that already integrates with WebKit very well when it comes to non-Apple ports.

The condition for malloc heap breakdown reporting to sysprof is that the WebKit browser needs to be profiled e.g. using:

sysprof-cli -f -- <BROWSER_COMMAND>

and the sysprof has to be in the latest version possible.

With the above, the memory usage statistics can then be inspected using the sysprof utility and look like in the image below:

In the case of sysprof, memory statistics in that case are just a minor addition to other powerful features that were well described in this blog post from Georges.

Caveats #

While malloc heap breakdown is very useful in some use cases — especially on embedded systems — there are a few problems with it.

First of all, compilation with -DENABLE_MALLOC_HEAP_BREAKDOWN=ON is not guarded by any continuous integration bots; therefore, the compilation issues are expected on the latest WebKit main. Fortunately, fixing the problems is usually straightforward. For a reference on what may be causing compilation problems usually, one should refer to 299555@main, which contains a full variety of fixes.

The second problem is that malloc heap breakdown uses WebKit’s debug heaps, and hence the memory usage patterns may be different just because system malloc is used.

The third, and final problem, is that malloc heap breakdown integration for non-Apple ports introduces some overhead as the allocations need to lock/unlock the mutex, and as statistics are stored in the memory as well.

Opportunities #

Although malloc heap breakdown can be considered fairly constrained, in the case of non-Apple ports, it gives some additional possibilities that are worth mentioning.

Because on non-Apple ports, the custom library is used to track allocations (as mentioned at the beginning of the Malloc heap breakdown on non-Apple ports section), it’s very easy to add more sophisticated tracking/debugging/reporting capabilities. The only file that requires changes in such a case is: Source/WTF/wtf/malloc_heap_breakdown/main.cpp.

Some examples of custom modifications include:

• adding different reporting mechanisms — e.g. writing to a file, or to some other tool,
• reporting memory usage with more details — e.g. reporting the per-memory-chunk statistics,
• dumping raw memory bytes — e.g. when some allocations are suspicious.
• altering memory in-place — e.g. to simulate memory corruption.

Summary #

While the presented malloc heap breakdown mechanism is a rather poor approximation of what industry standard tools offer, the main benefit of it is that it’s built into WebKit, and that in some rare use-cases (especially on embedded platforms), it’s the only way to perform any reasonable profiling.

In general, as a rule of thumb, it’s not recommended to use malloc heap breakdown unless all other methods have failed. In that sense, it should be considered a last resort approach. With that in mind, malloc heap breakdown can be seen as a nice mechanism complementing other tools in the toolbox.

Update on what happened in WebKit in the week from October 13 to October 20.

This week was calmer than previous week but we still had some meaningful updates. We had a Selenium update, improvements to how tile sizes are calculated, and a new Igalian in the list of WebKit committer!

Cross-Port 🐱

Graphics 🖼️

Community & Events 🤝

Safari Technology Preview Release 230 is now available for download for macOS Tahoe and macOS Sequoia. If you already have Safari Technology Preview installed, you can update it in System Settings under General → Software Update.

This release includes WebKit changes between: 300291@main…300986@main.

Animations

Resolved Issues

• Fixed animation-name resolution to correctly find matching @keyframes within tree-scoped and shadow DOM contexts. (300706@main) (156484228)

CSS

Resolved Issues

• Fixed incorrect handling of auto inline margins on grid items during track sizing that caused excessive vertical spacing in subgrids. (300422@main) (157638931)
• Fixed CSS anchor positioning to remember the last successful position option at ResizeObserver delivery time, aligning with the spec. (300890@main) (159225250)
• Fixed an issue where transitioning an element to display: none with transition-behavior: allow-discrete and CSS Anchor Positioning would repeatedly restart the transition. (300519@main) (160421419)
• Fixed the acceptable anchor algorithm in CSS Anchor Positioning to correctly consider inline elements as containing blocks. (300614@main) (160917762)
• Fixed CSS nesting to inline parent selectors when possible instead of always wrapping them in :is() to improve selector performance. (300297@main) (160927950)
• Fixed position-try-fallback resolution by treating names as tree-scoped references to properly search shadow DOM host scopes. (300333@main) (161081231)
• Fixed an issue where a <select> element with long <option> text caused horizontal scrolling when nested inside a flex item. (300684@main) (161563289)
• Fixed getComputedStyle to return numeric values (2) for orphans and widows instead of the internal auto value, ensuring the computed values correctly reflect the CSS specification. (300690@main) (161566631)
• Fixed column-count: 1 so that it now correctly creates a multi-column container per the CSS Multi-column Layout specification. (300787@main) (161611444)
• Fixed the calculation of anchor positions in vertical-rl multi-column layouts by correctly flipping coordinates in fragmented flows. (300807@main) (161616545)
• Fixed the order to try anchor position fallback options, such that the last successful position option is tried first, followed by the original style, and then the remaining options. (300909@main) (161714637)
• Fixed position-area handling to include the in-flow scrollable area of the initial containing block. (300921@main) (161741583)
• Fixed position-visibility: no-overflow to respond correctly to scrolling. (301211@main) (162173481)
• Fixed: Renamed position-area keywords from x-self-start, x-self-end, y-self-start, and y-self-end to self-x-start, self-x-end, self-y-start, and self-y-end respectively to align with updated CSSWG specifications.(301226@main) (162214793)

HTML

Resolved Issues

• Fixed an issue where navigating to :~:text fragments on dynamically generated pages did not highlight or scroll to the fragment. (300918@main) (150880542)

MathML

Resolved Issues

• Fixed rendering of unknown MathML elements so they now behave like mrow as required by the MathML Core specification. (300580@main) (148593275)

Media

Resolved Issues

• Fixed MediaRecorder to no longer fire erroneous error events when stopped immediately after track changes, aligning behavior with Chrome and closer to Firefox. (300682@main) (161124260)

Rendering

Resolved Issues

• Fixed incorrect clipping of position:fixed/sticky content during view transitions. (300561@main) (154886047)
• Fixed an issue that caused cropped flexbox elements to render incorrectly. (300433@main) (159638640)
• Fixed an issue where sticky elements at the edge of the viewport could disappear during rubber band scrolling. (300544@main) (160385933)
• Fixed flickering of elements with slow-painting content during view transitions. (300902@main) (160886647)
• Fixed an issue where elements with both opacity and CSS filter effects could render incorrectly. (300549@main) (161130683)
• Fixed an issue where elements with background images were not counted as contentful for Paint Timing. (300667@main) (161456094)

SVG

Resolved Issues

• Fixed an issue where stop-color incorrectly accepted hashless hex color values like 1234 by treating them as invalid to follow the spec. (300296@main) (119166640)
• Fixed an issue where SVG pattern tileImage could appear blurred or pixelated when zooming or printing. (300357@main) (159202567)
• Fixed SVGAElement so that its rel and relList attributes now affect navigation behavior, including proper handling of noopener, noreferrer, and the new opener value, aligning SVG links with HTMLAnchorElement behavior. (300462@main) (160724516)

Security

Resolved Issues

• Fixed parsing of require-trusted-types-for in CSP to ensure 'script' is only valid when followed by whitespace or end of buffer. (300770@main) (147760089)

Web API

New Features

• Added support for Largest Contentful Paint. (300834@main) (161705604)

Resolved Issues

• Fixed an issue where the first pointerdown event was lost after triggering a context menu by right-clicking. (300696@main) (84787733)
• Fixed Trusted Types to only verify event handler attributes for elements in the XHTML, SVG, and MathML namespaces, preventing incorrect checks on other namespaces. (300783@main) (147763139)
• Fixed an issue where navigate.back in the main frame would fail after a navigate.back in a child frame by properly clearing the provisional history item to allow correct back navigation. (301092@main) (158259024)
• Fixed NavigateEvent.sourceElement to correctly reference the submitting HTMLFormElement instead of null when a form is submitted. (301252@main) (160391355)
• Fixed EventCounts interface was not maplike. Enables use of methods such as .forEach(), keys(), and entries(). (300830@main) (160968888)
• Fixed an issue where mousemove events were still dispatched to removed mouseover targets instead of their parent element when the target was deleted. (300522@main) (161203639)
• Fixed missing pointerenter and mouseenter events when a child element moved under the mouse. (300564@main) (161362257)
• Fixed an issue where only one CSP violation report was sent for multiple enforced require-trusted-types-for directives. (300832@main) (161740298)
• Fixed Trusted Types incorrectly treating null or undefined policy return values as null instead of empty strings during createHTML, createScript, and createScriptURL operations. (300892@main) (161837641)

Web Extension

New Features

• Added support for browser.runtime.getVersion() to retrieve the extension version from its manifest. (300972@main) (161742137)

Web Inspector

Resolved Issues

• Fixed syntax highlighting for JavaScript features like template literals, private class elements, optional chaining, and others. (300332@main) (107619553)
• Fixed an issue where navigating the DOM tree using the keyboard would get stuck in a loop within certain subtrees. (300471@main) (159841729)
• Fixed an issue where adding DOM attributes or node siblings did not work correctly when using the actions from the context menu. (300752@main) (161577627)

Update on what happened in WebKit in the week from October 6 to October 13.

Another week with many updates in Temporal, the automated testing infrastructure is now running WebXR API tests; and WebKitGTK gets a fix for the janky Inspector resize while it drops support for libsoup 2. Last but not least, there are fresh releases of both the WPE and GTK ports including a security fix.

Cross-Port 🐱

Multimedia 🎥

JavaScriptCore 🐟

WebKitGTK 🖥️

Releases 📦️

Infrastructure 🏗️

Grid is a powerful, flexible tool that brings complex layouts to life. While it’s not new, and has been around for eight years, there’s so much to learn that it can still feel confusing and overwhelming to work with.

What’s the right mental model for thinking about Grid? What do all the new terms mean? What’s with that “/” anyway? If you have these questions and struggle to wrap your head around Grid, you’re not alone. There’s a lot of stuff to learn. Let’s see if we can unpack and explore it together, and make grid a little more comfortable to use.

When we think of laying out content on a webpage, we might start by listing out all of our elements in HTML. Then we use CSS to move and adjust them into place, changing their flow on the page, tweaking the space between them, until they’re finally in their right positions.

But with Grid, I want to try something different.

Instead of starting with that list of elements and shifting them around, we’re going to use a mental model to first focus on the container holding our elements.

Let’s think of our grid like we’re creating a brand new spreadsheet.

When you start a new document, depending on the software, you might have to first declare that it’s going to be a spreadsheet. Similarly, to use Grid we need to declare that the display for our container will be grid, like this:

.products {
  display: grid;
}

Before we put our data into a spreadsheet, we first think about what that spreadsheet will look like. We’ll probably start with the columns — what columns do we need to hold our data? If we’re working with a list of products, will the columns contain prices, discount codes, descriptions, launch dates?

Then we’ll think about the rows — how many rows of data do we need to contain everything? Once we have a sense of these columns and rows, we can decide where to place our data.

Similarly, our grid comes with columns and rows that we determine upfront. If I’m working on a grid of products, I might think about how many products I have and how that translates to columns and rows. I have six products, so I’m going to have three columns and two rows. And I’m going to throw in a grid-gap so I have some cushion. That should look good.

Here’s what that code might look like:

.products {
   display: grid;
   grid-template-columns: 1fr 1fr 1fr;
   grid-template-rows: 1fr 1fr;
   grid-gap: 2em;
}

Let’s dissect this code a bit further.

Here, I’m using fr for my values, which stands for fractional unit, and tells my CSS to divide the space up into fractions. While I don’t know exactly how wide I want each column to be, and that width should change anyway when the viewport width changes, I know that I want three equal columns. So, using my fr, I can divide up the width into three equal fractional units: 1fr 1fr 1fr.

Let’s now move on to something that might not be immediately intuitive. Why is it called grid-template-columns anyway? Why not just grid-columns? What does “template” mean in this context?

This was one of the new vocabulary of Grid that tripped me up when I first saw it. I constantly had to look up the syntax for setting columns and rows.

When it comes to columns and rows, there are actually several Grid properties that reference them.

The first, grid-template-columns and grid-template-rows reference the columns and rows on the container, the top-level element of our little grid world. The word “template” is meant to reference a sort of blueprint we’re creating — at the highest level, we are defining a template that the children within our container will adhere to. That template outlines the structure and creates the rules of our grid.

The second pair of properties is grid-row and grid-column. Unlike our first properties, these don’t refer to our container. Instead, we drop down to the children. These properties allow us to place individual items into certain rows and columns. I’ll show you how a bit later.

And the third set of properties are the auto properties: grid-auto-rows and grid-auto-columns. These allow you to define the size of any extra rows and columns for content you might not have planned for. So in my products example, I may start with six products, but since my team is always working on new ones, I might want to prepare for a future where I add a few more products. In that case, if I wanted them to take the same 1fr of space, I would set that property to 1fr , like this:

.products {
   display: grid;
   grid-template-columns: 1fr 1fr 1fr;
   grid-template-rows: 1fr 1fr;
   grid-gap: 2em;
   grid-auto-rows: 1fr;
}

Now let’s get back to my grid.

Just like I would think about my rows and columns in my spreadsheet, I’ve thought about and determined the rows and columns of my grid. Let’s explore other elements of our mental model.

In my spreadsheet, my rows and columns are separated by lines. In Grid, these are called my grid lines. Separating my columns are column grid lines, and separating my rows are row grid lines.

While we know that the cells moving from left to right (or right to left) are my rows and the cells going up and down are my columns, rows and columns are more generically known as “tracks”.

And if I have multiple cells grouped together spanning different rows and columns, that’s a “grid area.”

That’s some of the introductory vocabulary to give you a foundation of how to think about grid. Now that you know what things are called, let’s try actually laying out come products and see what happens.

I started with six products, but I don’t think I want to display all six right now. Instead, I’m going to feature my two best sellers, have a section for add-ons, and include a testimonial at the bottom. The design I have in mind might look something like this:

Just like how we start our spreadsheet by thinking about the columns and rows, I need to create my grid and start with considering the columns and rows I’ll need, creating a template that my individual elements will use.

Looking at this mock up, I can see that I have three columns and two rows. But there’s some merging going on. Just like a spreadsheet can have merged cells, I see that one of my “cells,” the Add-ons, actually spans two rows. And my other “cell,” the Testimonial, spans two columns.

But before we implement this design, let’s see what Grid does all on its own. If we just designate our template rows and columns and do nothing else, where do our elements end up?

Here’s the code we’re starting with:

.products {
   display: grid;
   grid-template-columns: 1fr 1fr 1fr;
   grid-template-rows: 1fr 1fr;
   grid-gap: 2em;
}

That’ll get us the following output:

Not too bad. The default behavior actually got us pretty far. Without us explicitly telling Grid where to put each element, it automatically placed the elements in the next available “cell” in our Grid, and when it ran out of column space, it wrapped around to the second row. Pretty close! This is one of the things that makes Grid so powerful — it offers a lot out of the box without us needed to be explicit.

Now we need to add the code to create the spanning action we have in our design.

This next step takes a little bit of upfront planning. Before I start merging and spanning things, I need to first establish my grid lines. But I’m going to take things a step further: I’m also going to number them.

This is where our grid and the mental model of a spreadsheet start to differ. If I were to describe where Product 1 is located in a spreadsheet, I would describe it as being in the first column and the first row. In fact, in a spreadsheet, it’s these rows and columns that have labels. But in Grid, the most common way to position elements actually isn’t by using the rows and columns at all. Instead, we use the numbered grid lines to dictate where an element goes. We don’t say “the first column,” we say “between the first and second column grid lines.”

If we compare our mockup with our output, there are only two elements whose placement we need to update: Add-ons and Testimonials. Let’s look at Add-ons first.

Here, we want to keep the default behavior of the columns — it should stay between the 3 and 4 column grid lines. But we want the height to span both rows. Put in grid line lingo, we want the element to start at row grid line 1 and end at row grid line 3. Let’s write the code to make that happen.

.add-on {
   grid-row: 1/3;
}

Let’s see what that looks like.

Something interesting happened! We got the spanning across rows we wanted, but now my Add-on section has jumped to first place. That’s because Grid prioritized explicitly positioned elements over implicitly positioned ones. Since right now, Add-on is the only element with any kind of explicit positioning, it gets priority positioning in my grid. But I don’t want that. So I need to add an explicit column positioning to get it back where I want it. Let’s do that now.

Since we’re positioning based on grid lines, we’re going to explicitly put our Add-on between the 3 and 4 column grid lines, like this:

.add-on {
   grid-row: 1/3;
   grid-column: 3/4;
}

That’ll get us this output:

Wonderful. Three elements placed, one more to go.

Here, we’re going to use grid lines once again. We’re going to have my Testimonial section start at row grid line 1 and go all the way to row grid line 3. Let’s code that now:

.testimonial {
   grid-column: 1/3;
}

And we get this output:

And just like that, we have our elements laid out just like our mockup.

We spent a lot of time in this article laying the foundation for Grid, establishing some important vocabulary that we later used to implement our own grid design. The biggest takeaway here, and the concept that tripped me up the most when I first learned Grid, is the importance of using those grid lines to place and span elements.

But that isn’t the only way to place items. You can also use something called grid areas that might feel a little more intuitive. We’ll cover that next.

If you found this helpful or have more feedback, you can share your feedback with me, Saron Yitbarek, on BlueSky, or reach out to our other evangelists — Jon Davis, on Bluesky / Mastodon, and Jen Simmons, on Bluesky / Mastodon. You can also follow WebKit on LinkedIn. If you find a bug or problem, please file a WebKit bug report.

Update on what happened in WebKit in the week from September 29 to October 6.

Another exciting weekful of updates, this time we have a number of fixes on MathML, content secutiry policy, and Aligned Trusted types, public API for WebKitWebExtension has finally been added, and fixed enumeration of speaker devices. In addition to that, there's ongoing work to improved compatibility for broken AAC audio streams in MSE, a performance improvement to text rendering with Skia was merged, and fixed multi-plane DMA-BUF handling in WPE. Last but not least, The 2026 edition of the Web Engines Hackfest has been announced! It will take place from June 15th to the 17th.

Cross-Port 🐱

Multimedia 🎥

JavaScriptCore 🐟

Graphics 🖼️

WPE WebKit 📟

WPE Platform API 🧩

Community & Events 🤝

The rise of e-commerce in the past decade changed the way customers interact with businesses online, leading to new innovations and improved user experiences. But while these interactions made leaps, one area that’s remained generally analog is online identity verification. A common form of online verification is for a website to ask users to take a photo of a government-issued ID for verification. This requires people to locate their driver’s license or passport, find a suitable background to capture a clear image of that ID and then upload it. Often, to help ensure that the person uploading the ID is the same person to whom the identity card belongs, people are asked to capture a selfie with it that leads to increased abandonment rates. Beyond that, businesses have to rely on expensive and complex systems to verify these uploaded photos which come with limited privacy protections and expose users to an increased risk of identity theft.

Mobile IDs are changing this landscape, making online identity verification easier, more secure, and more private for everyone.

What Are Mobile IDs?

Mobile identity documents are secure, electronic versions of government-issued or verified credentials (like driver’s licenses, passports, or ID cards) that can be stored and presented on a smartphone or other device. One important format for mobile identity documents are called mdocs.

Mobile documents, also known as mDocs are defined by the ISO/IEC 18013-5 and ISO/IEC 18013-7 standard, which allow for the secure presentment of digital identity documents in-person and online. The standard also provides for multiple security and privacy capabilities including:

• Data minimization: Allows relying parties to request only the information that is needed to complete a transaction.
• Identity data integrity and anti-forgery: Cryptographically signed to prevent tampering and reduce the risk of identity theft during presentment.
• Device binding: use a device signature to protect against the cloning of an ID and the replay of an identity presentation.
• User data confidentiality: Session encryption helps ensure that all personally identifiable information (PII) exchanged between the mdoc and the relying party is encrypted.

Introducing Web Support for the Digital Credentials API

Starting in Safari 26 on macOS 26, iOS 26, and iPadOS 26, we’re introducing support for the W3C’s Digital Credentials API. This means websites can now request mobile IDs from Apple Wallet and third party Wallets directly from Safari and other WebKit-based browsers across your iPhone, iPad, Mac, and other devices.

How It Works: A Real-World Example

An individual user would like to create a verified account with a car rental company to access preferred benefits such as a “skip-the-counter” service. As part of that verified account setup, the user is asked to verify their identity online by presenting their mobile ID.

On Safari on an iPhone:

1. User taps the “Verify Identity” button on the car rental’s website, which invokes the W3C’s Digital Credentials API.
2. A system UI appears allowing the user to select their preferred digital identity document (e.g., California ID in Apple Wallet).
3. A consent sheet displays the requested information, the website behind the request and whether the website intends to retain the requested information.
4. The requested information is sent to the website only after the user reviews and authorizes with their biometric or passcode to do so.

On Safari on a Mac or iPad:

The process is equally straightforward on macOS and iPadOS, and also relies on digital identity documents stored on your iPhone:

1. User taps the “Verify Identity” button on the car rental’s website, which invokes the W3C’s Digital Credentials API.
2. User is prompted to continue on their nearby iPhone that is signed into the same Apple account.
3. A consent sheet displays the requested information, the website behind the request and whether the website intends to retain the requested information.
4. The requested information is sent to the website only after the user reviews and authorizes with their biometric or passcode to do so.

On other platforms:

Users can also present their mobile IDs across other standards compliant non-Apple platforms:

1. User taps the “Verify Identity” button on the car rental’s website, which invokes the W3C’s Digital Credentials API.
2. The browser presents a QR code allowing the user to open the camera app on their iPhone and scan it to continue the identity verification process.
3. On the user’s iPhone, a consent sheet displays the requested information, the website behind the request and whether the website intends to retain the requested information.
4. The requested information is sent to the website only after the user authorizes with their biometric or passcode to do so.

How the Digital Credentials API works

Presenting your Digital Credential in Safari follows international standards, ensuring interoperability so that any browser or platform adopting the same standards can accept it:

• W3C Digital Credentials API for the JavaScript API.
• ISO/IEC 18013-7 Annex C and ISO/IEC 18013-5 for the request format.
• FIDO CTAP protocol for cross-platform flows.

Understanding the Technical Flow

1. User visits website: The request for a digital credential begins by some user action (e.g., pressing a “Verify Identity” button).
2. Server builds the request: The website’s server creates and signs a document request.
3. API call via JavaScript: The website calls the Digital Credentials API with the request.
4. OS handles routing: The browser forwards the request to the operating system.
5. Wallet selection UI: The OS determines available document sources (e.g., Apple Wallet or another app) and shows options to the user.
6. User authorization: The selected app handles user consent and authorization.
7. Encrypted response: An encrypted response (e.g., the request drivers license data) is sent back to the website.
8. Decrypt and validate: The website’s server decrypts and checks the values are as expected in the response (see “Security: Trust but Verify” below).

For Website Developers: Getting Started

As a website developer looking to implement digital identity verification, there are three main steps to consider:

1. Building the Request

The website server needs to create a properly formatted request containing:

• Document type: What kind of document the website is requesting (e.g., driver’s license in the mdoc format).
• Required elements: Specific information the website needs (e.g., full name, age, etc.) and whether the website intends to retain this information.
• Encryption parameters: Keys and nonces for encryption and signing the response.
• Authentication structures: A signed ReaderAuth structure that contains information to help identify the website to document providers.

For more information about building and signing the request, see Requesting a mobile document on the web.

2. Making the API Call

Below is a reference to the Digital Credentials API that can be used as a starting point for making your own calls:

const requestData = {
  protocol: "org-iso-mdoc",
  data // Built on your server
};

try {
  // Gets back an encrypted credential
  const credential = await navigator.credentials.get({
    mediation: "required",
    digital: {
       requests: [ requestData ]
    }
  });
  // Send credential back to server for decryption
  await fetch("/verify", { method: "POST", body: credential };
} catch (error) {
  // Handle errors or fallback to alternative methods
}

Important: this API call must be triggered by a user gesture (like a button click) to prevent a website from requesting a user’s identity without user interaction. Like with most modern JavaScript APIs, this check is done automatically by the browser.

3. Processing the Response

When the server receives the response, it needs to:

• Decrypt the response using its own private key.
• Validate the issuer’s signature through the IssuerAuth structure as defined in ISO/IEC 18013-5.
• Verify the authenticity of individual data elements by computing and comparing hash digests.
• Confirm the document came from the device it was issued to by validating the DeviceAuth structure.

The precise procedure required to perform these steps for processing the response is defined in ISO/IEC 18013-5.

Security: Trust but Verify

Security is paramount when dealing with identity information. The mdoc format as defined in ISO/IEC 18013-5 includes multiple layers of protection to detect, deter, and mitigate security risks:

Request Authentication: Websites identify themselves using certificates, so users know exactly who’s asking for their information.

End-to-End Encryption: Responses are encrypted using keys generated by the requesting website, preventing unauthorized actors from accessing the data.

Issuer Authentication: Digital signatures prove the authenticity of the identity data and prevent tampering.

Device Binding: mdoc authentication use a device signature to protect against the cloning of an ID and the replay of an identity presentation.

For App Developers

If you’re building an app that provides digital identity documents like a government licensing authority app or digital identity documents provider, you can integrate your app with Identity Document Services. This framework enables the presentment of identity documents on device and web browser support for the Digital Credentials API. Once authorized, a person can select your app during a identity documents request, where they can authorize the presentment of identification through a UI you create.

If instead you are an app developer who wants to request a digital identity document from your app, see Get started with the Verify with Wallet API.

Learn More and Next Steps

For a deeper dive into Digital Credentials implementation, we recommend watching this WWDC session: Streamline identity verification with Digital Credentials. This session covers how Digital Credentials can enhance online identity verification flows, explores website integration with the Digital Credentials API for requesting information from IDs in Wallet, and demonstrates how apps can provide their own identity documents for online verification using the IdentityDocumentServices framework.

Next Steps

Ready to get started? Here’s what you need to do:

For Web Developers:

• Register with Apple Business Connect to receive certificates for requesting IDs from Apple Wallet.
• Check with other document providers about their specific onboarding requirements.
• Review the ISO standards mentioned in this post for implementation details.
• Follow the Requesting a mobile document on the web guide to get started with your implementation.

For App Developers wanting to integrate with Identity Document Services:

• Implement the Identity Document Provider app extension in your iOS app.
• Study the IdentityDocumentServices framework documentation.
• Follow the Implementing as an identity document provider guide for native app integration.

Safari Technology Preview Release 229 is now available for download for macOS Tahoe and macOS Sequoia. If you already have Safari Technology Preview installed, you can update it in System Settings under General → Software Update.

This release includes WebKit changes between: 299669@main…300290@main.

Accessibility

Resolved Issues

• Fixed an issue where <label> elements targeted by aria-labelledby lost their LabelFor relationships after content changes. (300029@main) (158906980)

CSS

New Features

• Added support for text-decoration-line values spelling-error and grammar-error. (299919@main) (160494378)

Resolved Issues

• Fixed offsetParent to correctly return the fixed-position element’s containing block when it is not the viewport, such as a transformed element. (300097@main) (63739636)
• Fixed propagation of the body element’s writing-mode to the document element to match the CSS Writing Modes Level 4 specification. (300278@main) (149475070)
• Fixed @position-try so that revert-layer correctly only reverts the position-try origin instead of affecting other cascade origins. (299900@main) (154355428)
• Fixed the calculation of anchor() positions in right-to-left or vertical-rl containers to correctly account for left-side scrollbars. (300018@main) (155852237)
• Fixed baseline alignment for grid items by adding correct first baseline and last baseline row axis handling and properly accounting for baseline offsets. (299933@main). (155967278)
• Fixed issues with anchor() positioning during overscroll by clamping scroll positions, preventing unnecessary scroll recaptures, and ensuring anchor resolution stays in sync with layout changes. (300061@main) (159356009)
• Fixed an issue where anchor-positioned elements failed to update their position when the default anchor changed. (300142@main) (159899182)
• Fixed an issue where collapsed table rows subtracted border-spacing twice. (300023@main) (160542118)
• Fixed ::view-transition pseudo-element to use position: absolute instead of fixed to align with the updated specification. (300008@main) (160622000)
• Fixed container queries to allow container-name matching across the full flat tree, making container names tree-scoped in line with the CSS Conditional 5 specification. (300033@main) (160696378)
• Fixed handling of ::first-letter pseudo-elements to always force inline display unless floated.(300073@main) (160710650)
• Fixed the behavior of the nesting selector & directly inside @scope to correctly act like :where(:scope) for proper specificity handling. (300153@main) (160769736)

Events

Resolved Issues

• Fixed boundary pointer and mouse events not firing when the hit test target changed under a stationary pointer. (300277@main) (160147423)
• Fixed scroll event handling by consolidating scroll and scrollend into a single queue. (300239@main) (160936070)

Forms

Resolved Issues

• Fixed input fields with field-sizing: content so that larger placeholder text now correctly expands the height of the field by including the placeholder’s computed height. (299672@main) (123125836)
• Fixed painting for <input type="range"> sliders in right-to-left vertical block writing modes. (299690@main) (158567821)

JavaScript

Resolved Issues

• Fixed TypeError messages to be clearer in for-of loops. (300154@main) (159814766)

Networking

Resolved Issues

• Fixed the frozen iOS user agent string to report version 18_7 for iOS 26.1. (299818@main) (159902747)

SVG

New Features

• Added support for the onbegin event in the SVGAnimationElement IDL interface to align with the SVG animations specification. (299724@main) (130609424)
• Added support for the async attribute in SVGScriptElement to align behavior with HTMLScriptElement and other browsers. (299735@main) (151561361)
• Added support for the hreflang IDL attribute on SVGAElement to improve SVG link handling. (299704@main) (160133102)
• Added support for type attribute on SVG <a> element. (299770@main) (160222206)

Resolved Issues

• Fixed absolutely positioned SVG elements to correctly account for the containing block’s padding. (300054@main) (127608838)

Storage

Resolved Issues

• Fixed an issue where dedicated workers could inherit storage access from their parent document, preventing them from sending cross-site requests with cookies. (300120@main) (158814068)

Web API

New Features

• Added preview support for Event Timing API (Interaction to Next Paint) (300205@main) (160880698)

Resolved Issues

• Fixed window.opener being incorrectly set to null when a site-isolated iframe navigated to a new site, ensuring opener relationships persist across frame migrations. (299924@main) (117269418)
• Fixed scroll and scrollend events so they correctly fire on <input type="text"> elements instead of their inner elements. (300260@main) (157880733)
• Fixed the order of pointerup and boundary events so that pointerout and pointerover fire before pointerup when a child element is attached under the cursor. (https://commits.webkit.org/300273@main) (160913756)
• Fixed element.scrollTo and element.scrollBy so they correctly scroll text input fields by forwarding scroll operations to the inner text element. (300274@main) (160963921)

Deprecations

• Removed support for the non-standard “overflow” event. (281672@main) (71129110)

Web Inspector

Resolved Issues

• Fixed an issue where the Console truncated long string outputs. (300077@main) (124629101)

WebDriver

New Features

• Added support for new endpoints for setting storage access permission state and granting storage access to embedded frames for specific origins. (158263193)

Resolved Issues

• Fixed the navigate endpoint in WebDriver to properly validate URLs against the current browsing context and set the default readiness state to Interactive to align with the specification. (299780@main) (157031091)

WebGPU

New Features

• Added support for using GPUTexture objects as depth-stencil and resolve attachments in WebGPU render passes to match the specification. (299708@main) (159952306)

Update on what happened in WebKit in the week from September 22 to September 29.

Many news this week! We've got a performance improvement in the Vector implementation, a fix that makes a SVG attribute work similarly to HTML, and further advancements on WebExtension support. We also saw an update to WPE Android, the test infrastructure can now run WebXR tests, WebXR support in WPE Android, and a rather comprehensive blog post about the performance considerations of WPE WebKit with regards to the DOM tree.

Cross-Port 🐱

Graphics 🖼️

WPE WebKit 📟

WPE Platform API 🧩

WPE Android ↗ 🤖

Community & Events 🤝

Infrastructure 🏗️

When I first learned anchor positioning, I built a demo to help me figure out how it all worked. I had a goal of what I wanted and I was trying to figure out what properties I needed to make it happen. I had a profile picture, and, when you clicked on it, I wanted a menu to appear below it, but left aligned to the profile picture, like this:

I did some searching, and found that, using logical direction, the code to accomplish this looked like this:

.profile-menu {
  position-anchor: --profile-button;
  position: absolute;
  position-area: block-end span-inline-end;
}

Most of this code is good and fine, but there’s one bit that I found surprising and a bit unclear: span-inline-end. This property conveys a few things. The span tells me that I’m crossing columns and including more than one. Makes sense. The inline-end tells me that one of those columns is the inline-end . Great.

But there’s something missing.

If I’m spanning multiple columns, what’s the other column? Do we start all the way on the left and spanning three columns, or are we starting in the center and spanning just the two? Looking at this property, there’s no way to tell.

I’m wondering if there should be a change to how CSS works, so that, instead, developers would write center-span-inline-end. That spells it out clearly. You start at the center and you span over to inline-end. Every piece of the puzzle is there.

So the code above would be rewritten as this:

.profile-menu {
  position-anchor: --profile-button;
  position: absolute;
  position-area: block-end center-span-inline-end;
}

But with this new property, you get an extra word, making it a bit more verbose. So is that extra word worth the extra clarity to you? Or would you prefer to fill in the blank yourself and keep things concise?

This change to the spec is currently being considered, and we want to know if it’s a change you’d support or if you’d prefer things as they are.

Find me on BlueSky and let me know which you like better.

Designing performant web applications is not trivial in general. Nowadays, as many companies decide to use web platform on embedded devices, the problem of designing performant web applications becomes even more complicated. Typical embedded devices are orders of magnitude slower than desktop-class ones. Moreover, the proportion between CPU and GPU power is commonly different as well. This usually results in unexpected performance bottlenecks when the web applications designed with desktop-class devices in mind are being executed on embedded environments.

In order to help web developers approach the difficulties that the usage of web platform on embedded devices may bring, this blog post initiates a series of articles covering various performance-related aspects in the context of WPE WebKit usage on embedded devices. The coverage in general will include:

• introducing the demo web applications dedicated to showcasing use cases of a given aspect,
• benchmarking and profiling the WPE WebKit performance using the above demos,
• discussing the causes for the performance measured,
• inferring some general pieces of advice and rules of thumb based on the results.

This article, in particular, discusses the overhead of nodes in the DOM tree when it comes to layouting. It does that primarily by investigating the impact of idle nodes that introduce the least overhead and hence may serve as a lower bound for any general considerations. With the data presented in this article, it should be clear how the DOM tree size/depth scales in the case of embedded devices.

DOM tree #

Historically, the DOM trees emerging from the usual web page designs were rather limited in size and fairly shallow. This was the case as there were no reasons for them to be excessively large unless the web page itself had a very complex UI. Nowadays, not only are the DOM trees much bigger and deeper, but they also tend to contain idle nodes that artificially increase the size/depth of the tree. The idle nodes are the nodes in the DOM that are active yet do not contribute to any visual effects. Such nodes are usually a side effect of using various frameworks and approaches that conceptualize components or services as nodes, which then participate in various kinds of processing utilizing JavaScript. Other than idle nodes, the DOM trees are usually bigger and deeper nowadays, as there are simply more possibilities that emerged with the introduction of modern APIs such as Shadow DOM, Anchor positioning, Popover, and the like.

In the context of web platform usage on embedded devices, the natural consequence of the above is that web designers require more knowledge on how the particular browser performance scales with the DOM tree size and shape. Before considering embedded devices, however, it’s worth to take a brief look at how various web engines scale on desktop with the DOM tree growing in depth.

Desktop considerations #

To measure the impact of the DOM tree depth on the performance, the random-number-changing-in-the-tree.html?vr=0&ms=1&dv=0&ns=0 demo can be used to perform a series of experiments with different parameters.

In short, the above demo measures the average duration of a benchmark function run, where the run does the following:

• changes the text of a single DOM element to a random number,
• forces a full tree layout.

Moreover, the demo allows one to set 0 or more parent idle nodes for the node holding text, so that the layout must consider those idle nodes as well.

The parameters used in the URL above mean the following:

• vr=0 — the results are reported to the console. Alternatively (vr=1), at the end of benchmarking (~23 seconds), the result appears on the web page itself.
• ms=1 — the results are reported in “milliseconds per run”. Alternatively (ms=0), “runs per second” are reported instead.
• dv=0 — the idle nodes are using <span> tag. Alternatively, (dv=1) <div> tag is used instead.
• ns=N — the N idle nodes are added.

The idea behind the experiment is to check how much overhead is added as the number of extra idle nodes (ns=N) in the DOM tree increases. Since the browsers used in the experiments are not fair to compare due to various reasons, instead of concrete numbers in milliseconds, the results are presented in relative terms for each browser separately. It means that the benchmarking result for ns=0 serves as a baseline, and other results show the relative duration increase to that baseline result, where, e.g. a 300% increase means 3 times the baseline duration.

The results for a few mainstream browsers/browser engines (WebKit GTK MiniBrowser [09.09.2025], Chromium 140.0.7339.127, and Firefox 142.0) and a few experimental ones (Servo [04.07.2024] and Ladybird [30.06.2024]) are presented in the image below:

As the results show, trends among all the browsers are very close to linear. It means that the overhead is very easy to assess, as usually N times more idle nodes will result in N times the overhead. Moreover, up until 100-200 extra idle nodes in the tree, the overhead trends are very similar in all the browsers except for experimental Ladybird. That in turn means that even for big web applications, it’s safe to assume the overhead among the browsers will be very much the same. Finally, past the 200 extra idle nodes threshold, the overhead across browsers diverges. It’s very likely due to the fact that the browsers are not optimizing such cases as a result of a lack of real-world use cases.

All in all, the conclusion is that on desktop, only very large / specific web applications should be cautious about the overhead of nodes, as modern web browsers/engines are very well optimized for handling substantial amounts of nodes in the DOM.

Embedded device considerations #

When it comes to the embedded devices, the above conclusions are no longer applicable. To demonstrate that, a minimal browser utilizing WPE WebKit is used to run the demo from the previous section both on desktop and NXP i.MX8M Plus platforms. The latter is a popular choice for embedded applications as it has quite an interesting set of features while still having strong specifications, which may be compared to those of Raspberry Pi 5. The results are presented in the image below:

This time, the Y axis presents the duration (in milliseconds) of a single benchmark run, and hence makes it very easy to reason about overhead. As the results show, in the case of the desktop, 100 extra idle nodes in the DOM introduce barely noticeable overhead. On the other hand, on an embedded platform, even without any extra idle nodes, the time to change and layout the text is already taking around 0.6 ms. With 10 extra idle nodes, this duration increases to 0.75 ms — thus yielding 0.15 ms overhead. With 100 extra idle nodes, such overhead grows to 1.3 ms.

One may argue if 1.3 ms is much, but considering an application that e.g. does 60 FPS rendering, the time at application disposal each frame is below 16.67 ms, and 1.3 ms is ~8% of that, thus being very considerable. Similarly, for the application to be perceived as responsive, the input-to-output latency should usually be under 20 ms. Again, 1.3 ms is a significant overhead for such a scenario.

Given the above, it’s safe to state that the 20 extra idle nodes should be considered the safe maximum for embedded devices in general. In case of low-end embedded devices i.e. ones comparable to Raspberry Pi 1 and 2, the maximum should be even lower, but a proper benchmarking is required to come up with concrete numbers.

Inline vs block #

While the previous subsection demonstrated that on embedded devices, adding extra idle nodes as parents must usually be done in a responsible way, it’s worth examining if there are nuances that need to be considered as well.

The first matter that one may wonder about is whether there’s any difference between the overhead of idle nodes being inlines (display: inline) or blocks (display: block). The intuition here may be that, as idle nodes have no visual impact on anything, the overhead should be similar.

To verify the above, the demo from Desktop considerations section can be used with dv parameter used to control whether extra idle nodes should be blocks (1, <div>) or inlines (0, <span>). The results from such experiments — again, executed on NXP i.MX8M Plus — are presented in the image below:

While in the safe range of 0-20 extra idle nodes the results are very much similar, it’s evident that in general, the idle nodes of block type are actually introducing more overhead.

The reason for the above is that, for layout purposes, the handling of inline and block elements is very different. The inline elements sharing the same line can be thought of as being flattened within so called line box tree. The block elements, on the other hand, have to be represented in a tree.

To show the above visually, it’s interesting to compare sysprof flamegraphs of WPE WebProcess from the scenarios comprising 20 idle nodes and using either <span> or <div> for idle nodes:

idle <span> nodes:

idle <div> nodes:

The first flamegraph proves that there’s no clear dependency between the call stack and the number of idle nodes. The second one, on the other hand, shows exactly the opposite — each of the extra idle nodes is visible as adding extra calls. Moreover, each of the extra idle block nodes adds some overhead thus making the flamegraph have a pyramidal shape.

Whitespaces #

Another nuance worth exploring is the overhead of text nodes created because of whitespaces.

When the DOM tree is created from the HTML, usually a lot of text nodes are created just because of whitespaces. It’s because the HTML usually looks like:

<span>
  <span>
    (...)
  </span>
</span>

rather than:

<span><span>(...)</span></span>

which makes sense from the readability point of view. From the performance point of view, however, more text nodes naturally mean more overhead. When such redundant text nodes are combined with idle nodes, the net outcome may be that with each extra idle node, some overhead will be added.

To verify the above hypothesis, the demo similar to the above one can be used along with the above one to perform a series of experiments comparing the approach with and without redundant whitespaces: random-number-changing-in-the-tree-w-whitespaces.html?vr=0&ms=1&dv=0&ns=0. The only difference between the demos is that the w-whitespaces one creates the DOM tree with artificial whitespaces, simulating as-if it was written in the formatted document. The comparison results from the experiments run on NXP i.MX8M Plus are presented in the image below:

As the numbers suggest, the overhead of redundant text nodes is rather small on a per-idle-node basis. However, as the number of idle nodes scales, so does the overhead. Around 100 extra idle nodes, the overhead is noticeable already. Therefore, a natural conclusion is that the redundant text nodes should rather be avoided — especially as the number of nodes in the tree becomes significant.

Parents vs siblings #

The last topic that deserves a closer look is whether adding idle nodes as siblings is better than adding them as parent nodes. In theory, having extra nodes added as siblings should be better as the layout engine will have to consider them, yet it won’t mark them with a dirty flag and hence it won’t have to layout them.

As in other cases, the above can be examined using a series of experiments run on NXP i.MX8M Plus using the demo from Desktop considerations section and comparing against either random-number-changing-before-siblings.html?vr=0&ms=1&dv=0&ns=0 or random-number-changing-after-siblings.html?vr=0&ms=1&dv=0&ns=0 demo. As both of those yield similar results, any of them can be used. The results of the comparison are depicted in the image below:

The experiment results corroborate the theoretical considerations made above — idle nodes added as siblings indeed introduce less layout overhead. The savings are not very large from a single idle node perspective, but once scaled enough, they are beneficial enough to justify DOM tree re-organization (if possible).

Conclusions #

The above experiments mostly emphasized the idle nodes, however, the results can be extrapolated to regular nodes in the DOM tree. With that in mind, the overall conclusion to the experiments done in the former sections is that DOM tree size and shape has a measurable impact on web application performance on embedded devices. Therefore, web developers should try to optimize it as early as possible and follow the general rules of thumb that can be derived from this article:

1. Nodes are not free, so they should always be added with extra care.
2. Idle nodes should be limited to ~20 on mid-end and ~10 on low-end embedded devices.
3. Idle nodes should be inline elements, not block ones.
4. Redundant whitespaces should be avoided — especially with idle nodes.
5. Nodes (especially idle ones) should be added as siblings.

Although the above serves as great guidance, for better results, it’s recommended to do the proper browser benchmarking on a given target embedded device — as long as it’s feasible.

Also, the above set of rules is not recommended to follow on desktop-class devices, as in that case, it can be considered a premature optimization. Unless the particular web application yields an exceptionally large DOM tree, the gains won’t be worth the time spent optimizing.

Update on what happened in WebKit in the week from September 15 to September 22.

The first release in a new stable series is now out! And despite that, the work continues on WebXR, multimedia reliability, and WebExtensions support.

Cross-Port 🐱

Multimedia 🎥

Releases 📦️

Safari Technology Preview Release 228 is now available for download for macOS Tahoe and macOS Sequoia. If you already have Safari Technology Preview installed, you can update it in System Settings under General → Software Update.

This release includes WebKit changes between: 299100@main…299754@main.

Accessibility

New Features

• Added support for auto-expanding details and hidden=”until-found” elements for text searches done via assistive technologies. (299649@main) (159913471)

Resolved Issues

• Fixed an issue where VoiceOver reports the wrong radio count with a dynamically inserted radio option. (299585@main) (159221583)
• Fixed exposing content within dynamically expanded details elements in the accessibility tree. (299601@main) (159291226)

CSS

New Features

• Added support for :scope when the scoping root is :visited. (299560@main) (157588890)
• Added support for using color-mix() without a color space, defaulting to oklab. (299440@main) (159039709)
• Added support for display-p3-linear colors in CSS. (299381@main) (159579630)

Resolved Issues

• Fixed -webkit-user-select: none disabling find-in-page in Safari. (299624@main) (8081660)
• Fixed anchor position to only generate position options on base style. (299135@main) (158900076)
• Fixed out-of-flow box with no sibling ignoring align-content. (299107@main) (159097576)
• Fixed performance of :has(> .changed) .subject selectors. (299162@main) (159173631)
• Fixed handling of the ::first-line pseudo-element when floats prevent the initial line from containing inline content, ensuring correct styling is applied to the actual first formatted line. (299402@main) (159613287)

DOM

Resolved Issues

• Fixed an issue where command-clicking to open a link in a new tab navigates the current tab. (299537@main) (57216935)

Editing

Resolved Issues

• Fixed jumbled text when copy/pasting bidirectional text starting with left-to-right. (299176@main) (152236717)

Forms

Resolved Issues

• Fixed <select> element with long <option> text causing horizontal scrolling in grid or flex containers. (299631@main) (141633685)
• Fixed checkboxes and radio buttons missing borders in the filled state when “Increased Contrast” is enabled on macOS. (299269@main) (159226308)
• Fixed switch controls that may be sized incorrectly due to incorrect margins. (299543@main) (159729284)

JavaScript

New Features

• Added support for Wasm Memory buffer APIs. (299236@main) (159305098)
• Added support for Wasm JS String Builtins. (299455@main) (159679027)

Resolved Issues

• Fixed non-standard new Date(2024-12-3) yielding to an “Invalid Date” error. (299182@main) (141044926)
• Fixed poor error messages for destructing null or undefined values. (299244@main) (159340067)

Media

Resolved Issues

• Fixed western Arabic numbers being displayed in the video viewer instead of eastern Arabic numbers. (299348@main) (141281469)
• Fixed handling of null media accessibility caption profile. (299204@main) (159134245)
• Fixed hiding and resuming a webm video that sometimes causes a decoding error. (299366@main) (159508950)

PDF

Resolved Issues

• Fixed VoiceOver not recognizing the password form. (299180@main) (155907450)

Rendering

Resolved Issues

• Fixed list markers overlapping text in certain situations. (299671@main) (157054277)
• Fixed string search freezing when subject has large number (>1000). (292919@main) (159129919)
• Fixed position-visibility: anchors-visible visibility heuristic in certain situations. (299554@main) (159790886)
• Fixed buttons with box shadow being broken. (299603@main) (159888287)

SVG

Resolved Issues

• Fixed an issue where a dynamic change in a CSS property of an SVG element does not get reflected in the instances of the SVGElement. (299112@main) (98577657)
• Fixed an issue where <view> element was not applied to the root element. (299459@main) (159705519)

Service Worker

Resolved Issues

• Fixed an issue where service worker downloads are not being saved to Downloads folder. (299564@main) (154501503)

Web API

Resolved Issues

• Fixed preventing the javascript: protocol in navigation.navigate(). (299235@main) (158867866)
• Fixed an issue where click and auxclick event targeting does not follow pointer capture target override. (299567@main) (159477637)

Web Extensions

Resolved Issues

• Fixed an issue that caused the web page to crash when navigating to certain URLs with an extension enabled. (299173@main) (158180410)

Web Inspector

Resolved Issues

• Fixed an issue where accepting a completion suggestion for a shorthand property value would malform the combined value. (299157@main) (159107788)
• Fixed issue where searching on certain text fails to find matches. (299541@main) (159272725)

WebGPU

Resolved Issues

• Fixed incorrect handling of some PNG pixel formats in WebGPU. (299404@main) (158797747)

WebRTC

New Features

• Added encrypted field to RTCRtpHeaderExtensionParameters. (299356@main) (159279401)

Resolved Issues

• Fixed camera indicator staying enabled even after ending a meeting/access to camera. (299161@main) (152962650)

Update on what happened in WebKit in the week from September 8 to September 15.

The JavaScriptCore implementation of Temporal continues to be polished, as does SVGAElement, and WPE and WebKitGTK accessibility tests can now run (but they are not passing yet).

Cross-Port 🐱

JavaScriptCore 🐟

Update on what happened in WebKit in the week from September 1 to September 8.

In this week's installment of the periodical, we have better spec compliance of JavaScriptCore's implementation of Temporal, an improvement in how gamepad events are handled, WPE WebKit now implements a helper class which allows test baselines to be aligned with other ports, and finally, an update on recent work on Sysprof.

Cross-Port 🐱

JavaScriptCore 🐟

WPE WebKit 📟

Community & Events 🤝

Last year the Webkit project started to integrate its tracing routines with Sysprof. Since then, the feedback I’ve received about it is that it was a pretty big improvement in the development of the engine! Yay.

People started using Sysprof to have insights about the internal states of Webkit, gather data on how long different operations took, and more. Eventually we started hitting some limitations in Sysprof, mostly in the UI itself, such as lack of correlational and visualization features.

Earlier this year a rather interesting enhancement in Sysprof was added: it is now possible to filter the callgraph based on marks. What it means in practice is, it’s now possible to get statistically relevant data about what’s being executed during specific operations of the app.

In parallel to WebKit, recently Mesa merged a patch that integrates Mesa’s tracing routines with Sysprof. This brought data from yet another layer of the stack, and it truly enriches the profiling we can do on apps. We now have marks from the DRM vblank event, the compositor, GTK rendering, WebKit, Mesa, back to GTK, back to the compositor, and finally the composited frame submitted to the kernel. A truly full stack view of everything.

So, what’s the catch here? Well, if you’re an attentive reader, you may have noticed that the marks counter went from this last year:

To this, in March 2025:

And now, we’re at this number:

I do not jest when I say that this is a significant number! I mean, just look at this screenshot of a full view of marks:

Naturally, this is pushing Sysprof to its limits! The app is starting to struggle to handle such massive amounts of data. Having so much data also starts introducing noise in the marks – sometimes, for example, you don’t care about the Mesa marks, or the WebKit marks, of the GLib marks.

Hiding Marks

The most straightforward and impactful improvement that could be done, in light of what was explained above, was adding a way to hide certain marks and groups.

Sysprof heavily uses GListModels, as is trendy in GTK4 apps, so marks, catalogs, and groups are all considered lists containing lists containing items. So it felt natural to wrap these items in a new object with a visible property, and filter by this property, pretty straightforward.

Except it was not

Turns out, the filtering infrastructure in GTK4 did not support monitoring items for property changes. After talking to GTK developers, I learned that this was just a missing feature that nobody got to implementing. Sounded like a great opportunity to enhance the toolkit!

It took some wrestling, but it worked, the reviews were fantastic and now GtkFilterListModel has a new watch-items property. It only works when the the filter supports monitoring, so unfortunately GtkCustomFilter doesn’t work here. The implementation is not exactly perfect, so further enhancements are always appreciated.

So behold! Sysprof can now filter marks out of the waterfall view:

Counters

Another area where we have lots of potential is counters. Sysprof supports tracking variables over time. This is super useful when you want to monitor, for example, CPU usage, I/O, network, and more.

Naturally, WebKit has quite a number of internal counters that would be lovely to have in Sysprof to do proper integrated analysis. So between last year and this year, that’s what I’ve worked on as well! Have a look:

Unfortunately it took a long time to land some of these contributions, because Sysprof seemed to be behaving erratically with counters. After months fighting with it, I eventually figured out what was going on with the counters, and wrote the patch with probably my biggest commit message this year (beat only by few others, including a literal poem.)

Wkrictl

WebKit also has a remote inspector, which has stats on JavaScript objects and whatnot. It needs to be enabled at build time, but it’s super useful when testing on embedded devices.

I’ve started working on a way to extract this data from the remote inspector, and stuff this data into Sysprof as marks and counters. It’s called wkrict. Have a look:

This is far from finished, but I hope to be able to integrate this when it’s more concrete and well developed.

Future Improvements

Over the course of an year, the WebKit project went from nothing to deep integration with Sysprof, and more recently this evolved into actual tooling built around this integration. This is awesome, and has helped my colleagues and other contributors to contribute to the project in ways it simply wasn’t possible before.

There’s still *a lot* of work to do though, and it’s often the kind of work that will benefit everyone using Sysprof, not only WebKit. Here are a few examples:

• Integrate JITDump symbol resolution, which allows profiling the JavaScript running on webpages. There’s ongoing work on this, but needs to be finished.
• Per-PID marks and counters. Turns out, WebKit uses a multi-process architecture, so it would be better to redesign the marks and counters views to organize things by PID first, then groups, then catalogs.
• A new timeline view. This is strictly speaking a condensed waterfall view, but it makes it more obvious the relationship between “inner” and “outer” marks.
• Performance tuning in Sysprof and GTK. We’re dealing with orders of magnitude more data than we used to, and the app is starting to struggle to keep up with it.

Some of these tasks involve new user interfaces, so it would be absolutely lovely if Sysprof could get some design love from the design team. If anyone from the design team is reading this, we’d love to have your help

Finally, after all this Sysprof work, Christian kindly offered me to help co-maintain the project, which I accepted. I don’t know how much time and energy I’ll be able to dedicate, but I’ll try and help however I can!

I’d like to thank Christian Hergert, Benjamin Otte, and Matthias Clasen for all the code reviews, for all the discussions and patience during the influx of patches.

This article is a continuation of the series on damage propagation. While the previous article laid some foundation on the subject, this one discusses the cost (increased CPU and memory utilization) that the feature incurs, as this is highly dependent on design decisions and the implementation of the data structure used for storing damage information.

From the perspective of this article, the two key things worth remembering from the previous one are:

• The damage propagation is an optional WPE/GTK WebKit feature that — when enabled — reduces the browser’s GPU utilization at the expense of increased CPU and memory utilization.
• On the implementation level, the damage is almost always a collection of rectangles that cover the changed region.

The damage information #

Before diving into the problem and its solutions, it’s essential to understand basic properties of the damage information.

The damage nature #

As mentioned in the section about damage of the previous article, the damage information describes a region that changed and requires repainting. It was also pointed out that such a description is usually done via a collection of rectangles. Although sometimes it’s better to describe a region in a different way, the rectangles are a natural choice due to the very nature of the damage in the web engines that originates from the box model.

A more detailed description of the damage nature can be inferred from the Pipeline details section of the previous article. The bottom line is, in the end, the visual changes to the render tree yield the damage information in the form of rectangles. For the sake of clarity, such original rectangles may be referred to as raw damage.

In practice, the above means that it doesn’t matter whether, e.g. the circle is drawn on a 2D canvas or the background color of some block element changes — ultimately, the rectangles (raw damage) are always produced in the process.

Approximating the damage #

As the raw damage is a collection of rectangles describing a damaged region, the geometrical consequence is that there may be more than one set of rectangles describing the same region. It means that raw damage could be stored by a different set of rectangles and still precisely describe the original damaged region — e.g. when raw damage contains more rectangles than necessary. The example of different approximations of a simple raw damage is depicted in the image below:

Changing the set of rectangles that describes the damaged region may be very tempting — especially when the size of the set could be reduced. However, the following consequences must be taken into account:

• The damaged region could shrink when some damaging information would be lost e.g. if too many rectangles would be removed.
• The damaged region could expand when some damaging information would be added e.g. if too many or too big rectangles would be added.

The first consequence may lead to visual glitches when repainting. The second one, however, causes no visual issues but degrades performance since a larger area (i.e. more pixels) must be repainted — typically increasing GPU usage. This means the damage information can be approximated as long as the trade-off between the extra repainted area and the degree of simplification in the underlying set of rectangles is acceptable.

The approximation mentioned above means the situation where the approximated damaged region covers the original damaged region entirely i.e. not a single pixel of information is lost. In that sense, the approximation can only add extra information. Naturally, the lower the extra area added to the original damaged region, the better.

The approximation quality can be referred to as damage resolution, which is:

• low — when the extra area added to the original damaged region is significant,
• high — when the extra area added to the original damaged region is small.

The examples of low (left) and high (right) damage resolutions are presented in the image below:

The problem #

Given the description of the damage properties presented in the sections above, it’s evident there’s a certain degree of flexibility when it comes to processing damage information. Such a situation is very fortunate in the context of storing the damage, as it gives some freedom in designing a proper data structure. However, before jumping into the actual solutions, it’s necessary to understand the problem end-to-end.

The scale #

The Pipeline details section of the previous article introduced two basic types of damage in the damage propagation pipeline:

• layer damage — the damage tracked separately for each layer,
• frame damage — the damage that aggregates individual layer damages and consists of the final damage of a given frame.

Assuming there are L layers and there is some data structure called Damage that can store the damage information, it’s easy to notice that there may be L+1 instances of Damage present at the same time in the pipeline as the browser engine requires:

• L Damage objects for storing layer damage,
• 1 Damage object for storing frame damage.

As there may be a lot of layers in more complex web pages, the L+1 mentioned above may be a very big number.

The first consequence of the above is that the Damage data structure in general should store the damage information in a very compact way to avoid excessive memory usage when L+1 Damage objects are present at the same time.

The second consequence of the above is that the Damage data structure in general should be very performant as each of L+1 Damage objects may be involved into a considerable amount of processing when there are lots of updates across the web page (and hence huge numbers of damage rectangles).

To better understand the above consequences, it’s essential to examine the input and the output of such a hypothetical Damage data structure more thoroughly.

The input #

There are 2 kinds of Damage data structure input:

• other Damage,
• raw damage.

The Damage becomes an input of other Damage in some situations, happening in the middle of the damage propagation pipeline when the broader damage is being assembled from smaller chunks of damage. What it consists of depends purely on the Damage implementation.

The raw damage, on the other hand, becomes an input of the Damage always at the very beginning of the damage propagation pipeline. In practice, it consists of a set of rectangles that are potentially overlapping, duplicated, or empty. Moreover, such a set is always as big as the set of changes causing visual impact. Therefore, in the worst case scenario such as drawing on a 2D canvas, the number of rectangles may be enormous.

Given the above, it’s clear that the hypothetical Damage data structure should support 2 distinct input operations in the most performant way possible:

• add(Damage),
• add(Rectangle).

The output #

When it comes to the Damage data structure output, there are 2 possibilities either:

• other Damage,
• the platform API.

The Damage becomes the output of other Damage on each Damage-to-Damage append that was described in the subsection above.

The platform API, on the other hand, becomes the output of Damage at the very end of the pipeline e.g. when the platform API consumes the frame damage (as described in the pipeline details section of the previous article). In this situation, what’s expected on the output technically depends on the particular platform API. However, in practice, all platforms supporting damage propagation require a set of rectangles that describe the damaged region. Such a set of rectangles is fed into the platforms via APIs by simply iterating the rectangles describing the damaged region and transforming them to whatever data structure the particular API expects.

The natural consequence of the above is that the hypothetical Damage data structure should support the following output operation — also in the most performant way possible:

• forEachRectangle(...).

The problem statement #

Given all the above perspectives, the problem of designing the Damage data structure can be summarized as storing the input damage information to be accessed (iterated) later in a way that:

1. the performance of operations for adding and iterating rectangles is maximal (performance),
2. the memory footprint of the data structure is minimal (memory footprint),
3. the stored region covers the original region and has the area as close to it as possible (damage resolution).

With the problem formulated this way, it’s obvious that this is a multi-criteria optimization problem with 3 criteria:

1. performance (maximize),
2. memory footprint (minimize),
3. damage resolution (maximize).

Damage data structure implementations #

Given the problem of storing damage defined as above, it’s possible to propose various ways of solving it by implementing a Damage data structure. Before diving into details, however, it’s important to emphasize that the weights of criteria may be different depending on the situation. Therefore, before deciding how to design the Damage data structure, one should consider the following questions:

• What is the proportion between the power of GPU and CPU in the devices I’m targeting?
• What are the memory constraints of the devices I’m targeting?
• What are the cache sizes on the devices I’m targeting?
• What is the balance between GPU and CPU usage in the applications I’m going to optimize for?
  • Are they more rendering-oriented (e.g. using WebGL, Canvas 2D, animations etc.)?
  • Are they more computing-oriented (frequent layouts, a lot of JavaScript processing etc.)?

Although answering the above usually points into the direction of specific implementation, usually the answers are unknown and hence the implementation should be as generic as possible. In practice, it means the implementation should not optimize with a strong focus on just one criterion. However, as there’s no silver bullet solution, it’s worth exploring multiple, quasi-generic solutions that have been researched as part of Igalia’s work on the damage propagation, and which are the following:

• Damage storing all input rects,
• Bounding box Damage,
• Damage using WebKit’s Region,
• R-Tree Damage,
• Grid-based Damage.

All of the above implementations are being evaluated along the 3 criteria the following way:

1. Performance
   • by specifying the time complexity of add(Rectangle) operation as add(Damage) can be transformed into the series of add(Rectangle) operations,
   • by specifying the time complexity of forEachRectangle(...) operation.
2. Memory footprint
   • by specifying the space complexity of Damage data structure.
3. Damage resolution
   • by subjectively specifying the damage resolution.

Damage storing all input rects #

The most natural — yet very naive — Damage implementation is one that wraps a simple collection (such as vector) of rectangles and hence stores the raw damage in the original form. In that case, the evaluation is as simple as evaluating the underlying data structure.

Assuming a vector data structure and O(1) amortized time complexity of insertion, the evaluation of such a Damage implementation is:

1. Performance
   • insertion is O(1) ✅
   • iteration is O(N) ❌
2. Memory footprint
   • O(N) ❌
3. Damage resolution
   • perfect ✅

Despite being trivial to implement, this approach is heavily skewed towards the damage resolution criterion. Essentially, the damage quality is the best possible, yet the expense is a very poor performance and substantial memory footprint. It’s because a number of input rects N can be a very big number, thus making the linear complexities unacceptable.

The other problem with this solution is that it performs no filtering and hence may store a lot of redundant rectangles. While the empty rectangles can be filtered out in O(1), filtering out duplicates and some of the overlaps (one rectangle completely containing the other) would make insertion O(N). Naturally, such a filtering would lead to a smaller memory footprint and faster iteration in practice, however, their complexities would not change.

Bounding box Damage #

The second simplest Damage implementation one can possibly imagine is the implementation that stores just a single rectangle, which is a minimum bounding rectangle (bounding box) of all the damage rectangles that were added into the data structure. The minimum bounding rectangle — as the name suggests — is a minimal rectangle that can fit all the input rectangles inside. This is well demonstrated in the picture below:

As this implementation stores just a single rectangle, and as the operation of taking the bounding box of two rectangles is O(1), the evaluation is as follows:

1. Performance
   • insertion is O(1) ✅
   • iteration is O(1) ✅
2. Memory footprint
   • O(1) ✅
3. Damage resolution
   • usually low ⚠️

Contrary to the Damage storing all input rects, this solution yields a perfect performance and memory footprint at the expense of low damage resolution. However, in practice, the damage resolution of this solution is not always low. More specifically:

• in the optimistic cases (raw damage clustered), the area of the bounding box is close to the area of the raw damage inside,
• in the average cases, the approximation of the damaged region suffers from covering significant areas that were not damaged,
• in the worst cases (small damage rectangles on the other ends of a viewport diagonal), the approximation is very poor, and it may be as bad as covering the whole viewport.

As this solution requires a minimal overhead while still providing a relatively useful damage approximation, in practice, it is a baseline solution used in:

• Chromium,
• Firefox,
• WPE and GTK WebKit when UnifyDamagedRegions runtime preference is enabled, which means it’s used in GTK WebKit by default.

Damage using WebKit’s Region #

When it comes to more sophisticated Damage implementations, the simplest approach in case of WebKit is to wrap data structure already implemented in WebCore called Region. Its purpose is just as the name suggests — to store a region. More specifically, it’s meant to store rectangles describing region in an efficient way both for storage and for access to take advantage of scanline coherence during rasterization. The key characteristic of the data structure is that it stores rectangles without overlaps. This is achieved by storing y-sorted lists of x-sorted, non-overlapping rectangles. Another important property is that due to the specific internal representation, the number of integers stored per rectangle is usually smaller than 4. Also, there are some other useful properties that are, however, not very useful in the context of storing the damage. More details on the data structure itself can be found in the J. E. Steinhart’s paper from 1991 titled SCANLINE COHERENT SHAPE ALGEBRA published as part of Graphics Gems II book.

The Damage implementation being a wrapper of the Region was actually used by GTK and WPE ports as a first version of more sophisticated Damage alternative for the bounding box Damage. Just as expected, it provided better damage resolution in some cases, however, it suffered from effectively degrading to a more expensive variant bounding box Damage in the majority of situations.

The above was inevitable as the implementation was falling back to bounding box Damage when the Region’s internal representation was getting too complex. In essence, it was addressing the Region’s biggest problem, which is that it can effectively store N2 rectangles in the worst case due to the way it splits rectangles for storing purposes. More specifically, as the Region stores ledges and spans, each insertion of a new rectangle may lead to splitting O(N) existing rectangles. Such a situation is depicted in the image below, where 3 rectangles are being split into 9:

Putting the above fallback mechanism aside, the evaluation of Damage being a simple wrapper on top of Region is the following:

1. Performance
   • insertion is O(logN) ✅
   • iteration is O(N2) ❌
2. Memory footprint
   • O(N2) ❌
3. Damage resolution
   • perfect ✅

Adding a fallback, the evaluation is technically the same as bounding box Damage for N above the fallback point, yet with extra overhead. At the same time, for smaller N, the above evaluation didn’t really matter much as in such case all the performance, memory footprint, and the damage resolution were very good.

Despite this solution (with a fallback) yielded very good results for some simple scenarios (when N was small enough), it was not sustainable in the long run, as it was not addressing the majority of use cases, where it was actually a bit slower than bounding box Damage while the results were similar.

R-Tree Damage #

In the pursuit of more sophisticated Damage implementations, one can think of wrapping/adapting data structures similar to quadtrees, KD-trees etc. However, in most of such cases, a lot of unnecessary overhead is added as the data structures partition the space so that, in the end, the input is stored without overlaps. As overlaps are not necessarily a problem for storing damage information, the list of candidate data structures can be narrowed down to the most performant data structures allowing overlaps. One of the most interesting of such options is the R-Tree.

In short, R-Tree (rectangle tree) is a tree data structure that allows storing multiple entries (rectangles) in a single node. While the leaf nodes of such a tree store the original rectangles inserted into the data structure, each of the intermediate nodes stores the bounding box (minimum bounding rectangle, MBR) of the children nodes. As the tree is balanced, the above means that with every next tree level from the top, the list of rectangles (either bounding boxes or original ones) gets bigger and more detailed. The example of the R-tree is depicted in the Figure 5 from the Object Trajectory Analysis in Video Indexing and Retrieval Applications paper:

The above perfectly shows the differences between the rectangles on various levels and can also visually suggest some ideas when it comes to adapting such a data structure into Damage:

1. The first possibility is to make Damage a simple wrapper of R-Tree that would just build the tree and allow the Damage consumer to pick the desired damage resolution on iteration attempt. Such an approach is possible as having the full R-Tree allows the iteration code to limit iteration to a certain level of the tree or to various levels from separate branches. The latter allows Damage to offer a particularly interesting API where the forEachRectangle(...) function could accept a parameter specifying how many rectangles (at most) are expected to be iterated.
2. The other possibility is to make Damage an adaptation of R-Tree that conditionally prunes the tree while constructing it not to let it grow too much, yet to maintain a certain height and hence certain damage quality.

Regardless of the approach, the R-Tree construction also allows one to implement a simple filtering mechanism that eliminates input rectangles being duplicated or contained by existing rectangles on the fly. However, such a filtering is not very effective as it can only consider a limited set of rectangles i.e. the ones encountered during traversal required by insertion.

Damage as a simple R-Tree wrapper

Although this option may be considered very interesting, in practice, storing all the input rectangles in the R-Tree means storing N rectangles along with the overhead of a tree structure. In the worst case scenario (node size of 2), the number of nodes in the tree may be as big as O(N), thus adding a lot of overhead required to maintain the tree structure. This fact alone makes this solution have an unacceptable memory footprint. The other problem with this idea is that in practice, the damage resolution selection is usually done once — during browser startup. Therefore, the ability to select damage resolution during runtime brings no benefits while introduces unnecessary overhead.

The evaluation of the above is the following:

1. Performance
   • insertion is O(logMN) where M is the node size ✅
   • iteration is O(K) where K is a parameter and 0≤K≤N ✅
2. Memory footprint
   • O(N) ❌
3. Damage resolution
   • low to high ✅

Damage as an R-Tree adaptation with pruning

Considering the problems the previous idea has, the option with pruning seems to be addressing all the problems:

• the memory footprint can be controlled by specifying at which level of the tree the pruning should happen,
• the damage resolution (level of the tree where pruning happens) can be picked on the implementation level (compile time), thus allowing some extra implementation tricks if necessary.

While it’s true the above problems are not existing within this approach, the option with pruning — unfortunately — brings new problems that need to be considered. As a matter of fact, all the new problems it brings are originating from the fact that each pruning operation leads to the loss of information and hence to the tree deterioration over time.

Before actually introducing those new problems, it’s worth understanding more about how insertions work in the R-Tree.

When the rectangle is inserted to the R-Tree, the first step is to find a proper position for the new record (see ChooseLeaf algorithm from Guttman1984). When the target node is found, there are two possibilities:

1. adding the new rectangle to the target node does not cause overflow,
2. adding the new rectangle to the target node causes overflow.

If no overflow happens, the new rectangle is just added to the target node. However, if overflow happens i.e. the number of rectangles in the node exceeds the limit, the node splitting algorithm is invoked (see SplitNode algorithm from Guttman1984) and the changes are being propagated up the tree (see ChooseLeaf algorithm from Guttman1984).

The node splitting, along with adjusting the tree, are very important steps within insertion as those algorithms are the ones that are responsible for shaping and balancing the tree. For example, when all the nodes in the tree are full and the new rectangle is being added, the node splitting will effectively be executed for some leaf node and all its ancestors, including root. It means that the tree will grow and possibly, its structure will change significantly.

Due to the above mechanics of R-Tree, it can be reasonably asserted that the tree structure becomes better as a function of node splits. With that, the first problem of the tree pruning becomes obvious: tree pruning on insertion limits the amount of node splits (due to smaller node splits cascades) and hence limits the quality of the tree structure. The second problem — also related to node splits — is that with all the information lost due to pruning (as pruning is the same as removing a subtree and inserting its bounding box into the tree) each node split is less effective as the leaf rectangles themselves are getting bigger and bigger due to them becoming bounding boxes of bounding boxes (…) of the original rectangles.

The above problems become more visible in practice when the R-tree input rectangles tend to be sorted. In general, one of the R-Tree problems is that its structure tends to be biased when the input rectangles are sorted. Despite the further insertions usually fix the structure of the biased tree, it’s only done to some degree, as some tree nodes may not get split anymore. When the pruning happens and the input is sorted (or partially sorted) the fixing of the biased tree is much harder and sometimes even impossible. It can be well explained with the example where a lot of rectangles from the same area are inserted into the tree. With the number of such rectangles being big enough, a lot of pruning will happen and hence a lot of rectangles will be lost and replaced by larger bounding boxes. Then, if a series of new insertions will start inserting nodes from a different area which is partially close to the original one, the new rectangles may end up being siblings of those large bounding boxes instead of the original rectangles that could be clustered within nodes in a much more reasonable way.

Given the above problems, the evaluation of the whole idea of Damage being the adaptation of R-Tree with pruning is the following:

1. Performance
   • insertion is O(logMK) where M is the node size, K is a parameter, and 0<K≤N ✅
   • iteration is O(K) ✅
2. Memory footprint
   • O(K) ✅
3. Damage resolution
   • low to medium ⚠️

Despite the above evaluation looks reasonable, in practice, it’s very hard to pick the proper pruning strategy. When the tree is allowed to be taller, the damage resolution is usually better, but the increased memory footprint, logarithmic insertions, and increased iteration time combined pose a significant problem. On the other hand, when the tree is shorter, the damage resolution tends to be low enough not to justify using R-Tree.

Grid-based Damage #

The last, more sophisticated Damage implementation, uses some ideas from R-Tree and forms a very strict, flat structure. In short, the idea is to take some rectangular part of a plane and divide it into cells, thus forming a grid with C columns and R rows. Given such a division, each cell of the grid is meant to store at most one rectangle that effectively is a bounding box of the rectangles matched to that cell. The overview of the approach is presented in the image below:

As the above situation is very straightforward, one may wonder what would happen if the rectangle would span multiple cells i.e. how the matching algorithm would work in that case.

Before diving into the matching, it’s important to note that from the algorithmic perspective, the matching is very important as it accounts for the majority of operations during new rectangle insertion into the Damage data structure. It’s because when the matched cell is known, the remaining part of insertion is just about taking the bounding box of existing rectangle stored in the cell and the new rectangle, thus having O(1) time complexity.

As for the matching itself, it can be done in various ways:

• it can be done using strategies known from R-Tree, such as matching a new rectangle into the cell where the bounding box enlargement would be the smallest etc.,
• it can be done by maximizing the overlap between the new rectangle and the given cell,
• it can be done by matching the new rectangle’s center (or corner) into the proper cell,
• etc.

The above matching strategies fall into 2 categories:

• O(CR) matching algorithms that compare a new rectangle against existing cells while looking for the best match,
• O(1) matching algorithms that calculate the target cell using a single formula.

Due to the nature of matching, the O(CR) strategies eventually lead to smaller bounding boxes stored in the Damage and hence to better damage resolution as compared to the O(1) algorithms. However, as the practical experiments show, the difference in damage resolution is not big enough to justify O(CR) time complexity over O(1). More specifically, the difference in damage resolution is usually unnoticeable, while the difference between O(CR) and O(1) insertion complexity is major, as the insertion is the most critical operation of the Damage data structure.

Due to the above, the matching method that has proven to be the most practical is matching the new rectangle’s center into the proper cell. It has O(1) time complexity as it requires just a few arithmetic operations to calculate the center of the incoming rectangle and to match it to the proper cell (see the implementation in WebKit). The example of such matching is presented in the image below:

The overall evaluation of the grid-based Damage constructed the way described in the above paragraphs is as follows:

1. performance
   • insertion is O(1) ✅
   • iteration is O(CR) ✅
2. memory footprint
   • O(CR) ✅
3. damage resolution
   • low to high (depending on the CR) ✅

Clearly, the fundamentals of the grid-based Damage are strong, but the data structure is heavily dependent on the CR. The good news is that, in practice, even a fairly small grid such as 8x4 (CR=32) yields a damage resolution that is high. It means that this Damage implementation is a great alternative to bounding box Damage as even with very small performance and memory footprint overhead, it yields much better damage resolution.

Moreover, the grid-based Damage implementation gives an opportunity for very handy optimizations that improve memory footprint, performance (iteration), and damage resolution further.

As the grid dimensions are given a-priori, one can imagine that intrinsically, the data structure needs to allocate a fixed-size array of rectangles with CR entries to store cell bounding boxes.

One possibility for improvement in such a situation (assuming small CR) is to use a vector along with bitset so that only non-empty cells are stored in the vector.

The other possibility (again, assuming small CR) is to not use a grid-based approach at all as long as the number of rectangles inserted so far does not exceed CR. In other words, the data structure can allocate an empty vector of rectangles upon initialization and then just append new rectangles to the vector as long as the insertion does not extend the vector beyond CR entries. In such a case, when CR is e.g. 32, up to 32 rectangles can be stored in the original form. If at some point the data structure detects that it would need to store 33 rectangles, it switches internally to a grid-based approach, thus always storing at most 32 rectangles for cells. Also, note that in such a case, the first improvement possibility (with bitset) can still be used.

Summarizing the above, both improvements can be combined and they allow the data structure to have a limited, small memory footprint, good performance, and perfect damage resolution as long as there are not too many damage rectangles. And if the number of input rectangles exceeds the limit, the data structure can still fall-back to a grid-based approach and maintain very good results. In practice, the situations where the input damage rectangles are not exceeding CR (e.g. 32) are very common, and hence the above improvements are very important.

Overall, the grid-based approach with the above improvements has proven to be the best solution for all the embedded devices tried so far, and therefore, such a Damage implementation is a baseline solution used in WPE and GTK WebKit when UnifyDamagedRegions runtime preference is not enabled — which means it works by default in WPE WebKit.

Conclusions #

The former sections demonstrated various approaches to implementing the Damage data structure meant to store damage information. The summary of the results is presented in the table below:

While all the solutions have various pros and cons, the Bounding box and Grid-based Damage implementations are the most lightweight and hence are most useful in generic use cases.

On typical embedded devices — where CPUs are quite powerful compared to GPUs — both above solutions are acceptable, so the final choice can be determined based on the actual use case. If the actual web application often yields clustered damage information, the Bounding box Damage implementation should be preferred. Otherwise (majority of use cases), the Grid-based Damage implementation will work better.

On the other hand, on desktop-class devices – where CPUs are far less powerful than GPUs – the only acceptable solution is Bounding box Damage as it has a minimal overhead while it sill provides some decent damage resolution.

The above are the reasons for the default Damage implementations used by desktop-oriented GTK WebKit port (Bounding box Damage) and embedded-device-oriented WPE WebKit (Grid-based Damage).

When it comes to non-generic situations such as unusual hardware, specific applications etc. it’s always recommended to do a proper evaluation to determine which solution is the best fit. Also, the Damage implementations other than the two mentioned above should not be ruled out, as in some exotic cases, they may give much better results.

Update on what happened in WebKit in the week from August 25 to September 1.

The rewrite of the WebXR support continues, as do improvements when building for Android, along with smaller fixes in multimedia and standards compliance.

Cross-Port 🐱

Multimedia 🎥

Graphics 🖼️

WPE WebKit 📟

WPE Platform API 🧩

WPE Android ↗ 🤖

Update on what happened in WebKit in the week from August 18 to August 25.

This week continue improvements in the WebXR front, more layout tests passing, support for CSS's generic font family for math, improvements in the graphics stack, and an Igalia Chat episode!

Cross-Port 🐱

Graphics 🖼️

Community & Events 🤝

Update on what happened in WebKit in the week from August 11 to August 18.

This week we saw updates in WebXR support, better support for changing audio outputs, enabling of GLib API when building the JSCOnly port, improvements to damaging propagation, WPE platform enhancements, and more!

Cross-Port 🐱

Multimedia 🎥

JavaScriptCore 🐟

Graphics 🖼️

WPE WebKit 📟

WPE Platform API 🧩

Recently, I have been working on webkitgtk support for in-band text tracks in Media Source Extensions, so far just for WebVTT in MP4. Eventually, I noticed a page that seemed to be using a CEA-608 track - most likely unintentionally, not expecting it to be handled - so I decided to take a look how that might work. Take a look at the resulting PR here: https://github.com/WebKit/WebKit/pull/47763

Now, if you’re not already familiar with subtitle and captioning formats, particularly CEA-608, you might assume they must be straightforward, compared to audio and video. After all, its just a bit of text and some timestamps, right?

However, even WebVTT as a text-based format already provides lots of un- or poorly supported features that don’t mesh well with MSE - for details on those open questions, take a look at Alicia’s session on the topic: https://github.com/w3c/breakouts-day-2025/issues/14

Quick introduction to CEA-608 #

CEA-608, also known as line 21 captions, is responsible for encoding captions as a fixed-bitrate stream of byte pairs in an analog NTSC broadcast. As the name suggests, they are transmitted during the vertical blanking period, on line 21 (and line 284, for the second field) - imagine this as the mostly blank area “above” the visible image. This provides space for up to 4 channels of captioning, plus some additional metadata about the programming, though due to the very limited bandwidth, these capabilities were rarely used to their full extent.

While digital broadcasts provide captioning defined by its successor standard CEA-708, this newer format still provides the option to embed 608 byte pairs. This is still quite common, and is enabled by later standards defining a digital encoding, known as Caption Distribution Packets.

These are also what enables CEA-608 tracks in MP4.

Current issues, and where to go in the future #

The main issue I’ve encountered in trying to make CEA-608 work in an MSE context lies in its origin as a fixed-bitrate stream - there is no concept of cues, no defined start or end, just one continuous stream.

As WebKit internally understands only WebVTT cues, we rely on GStreamer’s cea608tott element for the conversion to WebVTT. Essentially, this element needs to create cues with well-defined timestamps, which works well enough if we have the entire stream present on disk.

However, when 608 is present as a track in an MSE stream, how do we tell if the “current” cue is continued in the next SourceBuffer? Currently, cea608tott will just wait for more data, and emit another cue once it encounters a line break, or its current line buffer fills up, but this also means the final cue will be swallowed, because there will never be “more data” to allow for that decision.

The solution would be to always cut off cues at SourceBuffer boundaries, so cues might appear to be split awkwardly to the viewer. Overall, this conversion to VTT won’t reproduce the captions as they were intended to be viewed, at least not currently. In particular, roll-up mode can’t easily be emulated using WebVTT.

The other issue is that I’ve assumed for the current patch that CEA-608 captions will be present as a separate MP4 track, while in practice they’re usually injected into the video stream, which will be harder to handle well.

Finally, there is the risk of breaking existing websites, that might have unintentionally left CEA-608 captions in, and don’t handle a surprise duplicate text track well.

Takeaway #

While this patch only provides experimental support so far, I feel this has given me valuable insight into how inband text tracks can work with various formats aside from just WebVTT. Ironically, CEA-608 even avoids some of WebVTT’s issues - there are no gaps or overlapping cues to worry about, for example.

Either way, I’m looking forward to improving on WebVTT’s pain points, and maybe adding other formats eventually!

Update on what happened in WebKit in the week from July 29 to August 11.

This update covers two weeks, including a deluge of releases and graphics work.

Cross-Port 🐱

Graphics 🖼️

WPE WebKit 📟

WPE Android ↗ 🤖

adb shell setprop debug.log.WPEWebKit all
adb shell setprop log.tag.WPEWebKit WARN

Releases 📦️

Update on what happened in WebKit in the week from July 21 to July 28.

This week the trickle of improvements to the graphics stack continues with more font handling improvements and tuning of damage information; plus the WPEPlatform Wayland backend gets server-side decorations with some compositors.

Cross-Port 🐱

Graphics 🖼️

WPE WebKit 📟

WPE Platform API 🧩