Bosch has been paying Collabora to work on WebKit-Clutter, the port of the WebKit rendering engine to the Clutter tool-kit. In one blog posting by Tomeu Vizoso, the greater video acceleration support is talked about, which in turn uses Clutter-GStreamer. The improvement here comes not by adopting some new video API but rather handling page composition on the GPU rather than within Cairo itself.
In another blog posting by Gustavo Noronha is talk about the new accelerated compositing support by WebKit-Clutter. This work also helps out the speed of animations within WebKit.