Video hardware acceleration is being worked on for WebKitGTK+ with composited video support.
WebKitGTK+ has been worked on a lot lately from Clutter acceleration
for the GTK+ version of the WebKit rendering engine to being ported to Wayland
Igalia has been working on accelerated composite video support with a goal of having a hardware-accelerated path for playback of 1080 high-definition video content. While they have 1080p video playback working with WebKitGTK+, they are waiting for improvements to land in GStreamer 1.2, the VA-API video acceleration back-end for GStreamer needs to be updated to the new API, there's some tearing issues to work through, and they are still working on CPU/GPU/memory optimizations.
For details on this composited video support work being done for WebKitGTK+ by Igalia can be found in this lengthy blog post