Something that crossed my mind, maybe you can help me answer. Are there cases where it would be easier to create a GTK application than a javascript application? This whole concept seems like it opens up a lot of opportunities for websites to provide rich interfaces, but then I think, wait, aren't people thinking of going the opposite direction? For instance, the high level GUI programming for Gnome Shell is in javascript.
It would be entirely possible to do both at the same time. There are javascript bindings for GTK. This isn't so much a workaround for javascript as a workaround for HTML.
And yes, there are absolutely cases where it would be easier to create a GTK application than an HTML application. You don't need to worry about choosing a widget library, for one, and placement is very straightforward.
I've been looking at Cappuccino since it does support stuff like tables with > 10.000 entries and 'complex' controls.
Now with GTK+ HTML5 support this might also be an avenue to explore.