Alexander Larsson, the GNOME developer behind this work, has commented that he has made some improvements to this HTML5 back-end and has now merged it into the GTK code-base. "Even thought the backend is not of production quality it is now good enough that I think its interesting for a larger audience to play around with."
The HTML5 back-end is called the GNOME Broadway back-end. Right now the back-end is only designed to work with the Mozilla Firefox 4.0 web-browser. There's a demonstration video of this new GTK+ feature below.
GTK+ 3.2 should officially land later in the year with this work and other changes. Read more in this GNOME blog post.