WebKitGTK+ is the version of the WebKit rendering engine deployed by numerous GNOME applications. Developers at Igalia have been porting this GNOME WebKit port to work on Wayland rather than just X11, but they have hit an issue along the way.
With WebKit2 there's a multi-process architecture where by two processes are rendering the web-page. This multi-process architecture works in X11 by using an offscreen XWindow and then using XComposite to push it to a pixmap and paint it to the screen. However, Igalia developers were unable to figure out how to share a Wayland surface between two processes to achieve the same multi-process rendering support.
Unable to achieve the multi-process rendering, Iago Toral of Igalia explained their work on the Wayland developers' list.
Fortunately, there is a way to obtain multi-process rendering, as was pointed out, and that's through nested compositing. Nested compositing is possible with the current state of Wayland, but lacking is an EGL Wayland API for providing buffers from a nested client to the host client surface without needing to be drawed an additional time.
Hopefully this WebKitGTK+ port to Wayland will come to fruition as it will help the GNOME desktop in being ported to this X11 and Mir alternative.