No, I think it was not windows only, and not as "fancy" or versatile, just something like gtk or Qt with built-in networking.

But this looks neat as well, I did not know that one nor found it during my search.

GTK3 has had an HTML5 rendering backend for a while now, but it does fall under the "streaming pixels" category: https://blogs.gnome.org/alexl/2010/11/23/gtk3-vs-html5/