The GTK+3 HTML5 back-end is being worked on by Red Hat's Alexander Larsson. This work has been underway for the past few weeks as is explained in this blog post and best summarized by this video below.
As mentioned on the GTK-devel mailing list, this HTML5 back-end work has been pushed upstream and is currently living under a GTK+3 branch under the name "Broadway" in Git. This back-end currently works with Mozilla Firefox 4.0, but other web-browsers will be supported eventually.
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