GTK+ 3.8.0 has been released ahead of this week's GNOME 3.8 desktop release. GTK+ 3.8 supports the Wayland 1.0 protocol, provides Broadway HTML5 server advancements, performance improvements, and much more.
The key features of GTK+ 3.8.0 is Wayland 1.0 stable support, support for the broadwayd server, improved themeing, better geometry management, touch improvements, support with the window manager for the frame synchronization protocol, and a variety of performance improvements.
Performance work for GTK+ 3.8 includes optimizing size allocation, performance of size groups improvements, text rendering improvements, reducing icon loading overhead, and CSS improvements.
Features of the HTML5 Broadway back-end for GTK+ are covered within this recent Phoronix post
More details on GTK+ 3.8.0 changes can be found from the gnome-announce-list