Moikka! Here's Adrián. I work for Igalia.

WPE WebKit 2.24

While WPE WebKit 2.24 has now been out for a couple of months, it includes over a year of development effort since our first official release, which means there is plenty to talk about. Let’s dive in!

API & ABI Stability

The public API for WPE WebKit has been essentially unchanged since the 2.22.x releases, and we consider it now stable and its version has been set to 1.0. The pkg-config modules for the main components have been updated accordingly and are now named wpe-1.0 (for libwpe), wpebackend-fdo-1.0 (the FDO backend), and wpe-webkit-1.0 (WPE WebKit itself).

Our plan for the foreseeable future is to keep the WPE WebKit API backwards-compatible in the upcoming feature releases. On the other hand, the ABI might change, but will be kept compatible if possible, on a best-effort basis.

Both API and ABI are always guaranteed to remain compatible inside the same stable release series, and we are trying to follow a strict “no regressions allowed” policy for patch releases. We have added a page in the Web site which summarizes the WPE WebKit release schedule and this API/ABI stability guarantee.

This should allow distributors to always ship the latest available point release in a stable series. This is something we always strongly recommend because almost always they include fixes for security vulnerabilities.


Web engines are security-critical software components, on which users rely every day for visualizing and manipulating sensitive information like personal data, medical records, or banking information—to name a few. Having regular releases means that we are able to publish periodical security advisories detailing the vulnerabilities fixed by them.

As WPE WebKit and WebKitGTK share a number of components with each other, advisories and the releases containing the corresponding fixes are published in sync, typically on the same day.

The team takes security seriously, and we are always happy to receive notice of security bugs. We ask reporters to act responsibly and observe the WebKit security policy for guidance.

Content Filtering

This new feature provides access to the WebKit internal content filtering engine, also used by Safari content blockers. The implementation is quite interesting: filtering rule sets are written as JSON documents, which are parsed and compiled to a compact bytecode representation, and a tiny virtual machine which executes it for every resource load. This way deciding whether a resource load should be blocked adds very little overhead, at the cost of a (potentially) slow initial compilation. To give you an idea: converting the popular EasyList rules to JSON results in a ~15 MiB file that can take up to three seconds to compile on ARM processors typically used in embedded devices.

In order to penalize application startup as little as possible, the new APIs are fully asynchronous and compilation is offloaded to a worker thread. On top of that, compiled rule sets are cached on disk to be reused across different runs of the same application (see WebKitUserContentFilterStore for details). Last but not least, the compiled bytecode is mapped on memory and shared among all the processes which need it: a browser with many tabs opened will practically use the same amount of memory for content filtering than one with a single Web page loaded. The implementation is shared by the GTK and WPE WebKit ports.

I had been interested in implementing support for content filtering even before the WPE WebKit port existed, with the goal of replacing the ad blocker in GNOME Web. Some of the code had been laying around in a branch since the 2016 edition of the Web Engines Hackfest, it moved from my old laptop to the current one, and I worked on it on-and-off while the different patches needed to make it work slowly landed in the WebKit repository—one of the patches went through as many as seventeen revisions! At the moment I am still working on replacing the ad blocker in Web—on my free time—which I expect will be ready for GNOME 3.34.

It’s All Text!

No matter how much the has evolved over the years, almost every Web site out there still needs textual content. This is one department where 2.24.x shines: text rendering.

Carlos García has been our typography hero during the development cycle: he single-handedly implemented support for variable fonts (demo), fixed our support for composite emoji (like 🧟‍♀️, composed of the glyphs “woman” and “zombie”), and improved the typeface selection algorithm to prefer coloured fonts for emoji. Additionally, many other subtle issues have been fixed, and the latest two patch releases include important fixes for text rendering.

Tip: WPE WebKit uses locally installed fonts as fallback. You may want to install at least one coloured font like Twemoji, which will ensure emoji glyphs can always be displayed.

API Ergonomics

GLib 2.44 added a nifty feature back in 2015: automatic cleanup of variables when they go out of scope using g_auto, g_autofree, and g_autoptr.

We have added the needed bits in the headers to allow their usage with the types from the WPE WebKit API. This enables developers to write code less likely to introduce accidental memory leaks because they do not need to remember freeing resources manually:

WebKitWebView* create_view (void)
    g_autoptr(WebKitWebContext) ctx = webkit_web_context_new ();
     * Configure "ctx" to your liking here. At the end of the scope (this
     * function), a g_object_unref(ctx) call will be automatically done.
    return webkit_web_view_new_with_context (ctx);

Note that this does not change the API (nor the ABI). You will need to build your applications with GCC or Clang to make use of this feature.

“Featurism” and “Embeddability”

Look at that, I just coined two new “technobabble” terms!

There are many other improvements which are shipping right now in WPE WebKit. The following list highlights the main user and developer visible features that can be found in the 2.24.x versions:

  • A new GObject based API for JavaScriptCore.
  • A fairly complete WebDriver implementation. There is a patch for supporting WPE WebKit in Selenium pending to be integrated. Feel free to vote 👍 for it to be merged.
  • WPEQt, which provides an idiomatic API similar to that of QWebView and allows embedding WPE WebKit as a widget in Qt/QML applications.
  • Support for the JPEG2000 image format. Michael Catanzaro has written about the reasoning behind this in his write-up about WebKitGTK 2.24.
  • Allow configuring the background of the WebKitWebView widget. Translucent backgrounds work as expected, which allows for novel applications like overlaying Web content on top of video streams.
  • An opt-in 16bpp rendering mode, which can be faster in some cases—remember to measure and profile in your target hardware! For now this only works with the RGB565 pixel format, which is the most common one used in embedded devices where 24bpp and 32bpp modes are not available.
  • Support for hole-punching using external media players. Note that at the moment there is no public API for this and you will need to patch the WPE WebKit code to plug your playback engine.

Despite all the improvements and features, still the main focus of WPE WebKit is providing an embeddable Web engine. Fear not: new features either are opt-in (e.g. 16bpp rendering), or disabled by default and add no overhead unless enabled (WebDriver, background color), or have no measurable impact at all (g_autoptr). Not to mention that many features can be even disabled at build time, bringing to the table smaller binaries and runtime footprint—but that would be a topic for another day.