Update on July 30, 2022: The WebKit project recently migrated to GitHub. The code for it can now be viewed at its new location (with iOS- and macOS-specific implementations). It is also possible to get an Atom feed of API changes.

When doing web development (or really any development on a complex enough platform), sometimes the best course of action for understanding puzzling behavior is to read the source. It's therefore been fortunate that Chrome/Blink, WebKit (though not Safari) and Firefox/Gecko are all open source (and often the source is easily searchable too).

One of exceptions has been mobile WebKit when accessed via a UIWebView on iOS. Though it's based on the same components (WebCore, JavaScriptCore, WebKit API layer) as its desktop counterpart, there is enough mobile-specific behavior (e.g. interaction with auto-complete and the on-screen keyboard) that source access would come in handy. Apple would periodically do code dumps on opensource.apple.com, but those only included the WebCore and JavaScriptCore components¹ and in any case there hasn't been one since iOS 6.1.

At WWDC, as part of iOS 8, Apple announced a modern WebKit API that would be unified between the Mac and iOS. Much of the (positive) reaction has been about the new API giving third-party apps access to faster, JITed, JavaScript execution. However, just as important to me is the fact that implementation of the new API is open source.

Besides browsing around the source tree, it's also possible to track its development more closely, via an RSS feed of commits. However, there are no guarantees that just because something is available in the trunk repository that it will also be available on the (presumed) branch that iOS 8 is being worked on. For example, [WKWebView evaluateJavaScript:completionHandler:] was added on June 10, but it didn't show up in iOS 8 until beta 3, released on July 7 (beta 2 was released on June 17). More recent changes, such as the ability to control selection granularity (added on June 26) have yet to show up². There don't seem to be any (header) changes³ that live purely in the iOS 8 SDK, so I'm inclined to believe that (at least at this stage) there's not much on-branch development, which is encouraging.

Many thanks to Anders, Benjamin, Mitz and all the other Apple engineers for doing all in the open.

1. IANAL, but my understanding is that WebCore and JavaScriptCore are LGPL-licensed (due to its KHTML heritage) and so modifications in shipping software have to distributed as source, while WebKit is BSD-licensed, and therefore doesn't have that requirement.
2. The selection granularity API eventually showed up in beta 4, which was released on July 21.
3. Modulo some munging done as part of the release process.