It's far from complete at this point, and as such, it'll be part of Gtk+ 3.2. Gtk+ 3.0 introduced support for multiple back-ends, and this new HTML5 gdk back-end makes use of that feature. After compiling Gtk+ with
–enable-x11-backend –enable-broadway-backend, you're almost good to go.
The second step is to enable web sockets in Firefox 4.0. This feature is disabled by default by Mozilla due to security concerns, but here's how to turn it on. Once that's done, you can enable the HTML5 back-end at runtime using the GDK_BACKEND enviroment variable.
The next step is to run a Gtk+-application, like so:
Alexander Larsson made a screencast of how this works.