while I'm not aware of anything that fits your description, you could have a look at the Mozilla Project's "Chromeless":
Instead building a whole new platform, we suggest that the web itself should be the platform. That a developer could design the browser using standard web technologies combined with a minimal set of new APIs to interact with the underlying operating system and control the application's user interface.
It's supposed to be a the-browser-is-the-OS thingy like (your, haha) Chrome OS.
Also have a look at Symphony OS with their Mezzo desktop environment, I don't know any details, but apparently they use some Mozilla base, too, so the desktop environment includes HTML, JS, even Flash.
Oh, and the whole issue gets turned upside down if you consider GTK3's HTML5 backend.