I've been tempted to use (the impressive) Cappuccino for a couple of apps but based on its lack of keyboard tab focus and performance on slower systems I decided against using it. I like to build simple, accessible apps.
Somebody--Sofa I think--was using it for back-end administration while presenting a more typical interface to public users, and that may be a better approach.
Perhaps you should check out NOLOH, http://www.noloh.com. It's similar to Cappuccino in some ways, yet very different in others. For instance, NOLOH is lightweight and on-demand. It also fails gracefully when JS is disabled in most cases, and is faster overall (this is based on our internal testing. Not trying to start a war). Notice how noloh.com loads instantly without any loading, or delay.
NOLOH also works with tab focus, and is more accessibility friendly. You can read more about our accessibility features at https://www.noloh.com/#/faqs/. We'll be posting major updates to HN in the next week or two, we last posted around a year ago, in case you haven't heard of us.
Somebody--Sofa I think--was using it for back-end administration while presenting a more typical interface to public users, and that may be a better approach.