I have an existing app (two-pane file manager) written in XUL. I use xulrunner/firefox to run it.
However, I fear that Mozilla's support of the underlying tech (XUL, XPCOM) is getting worse and worse, so I am thinking about rewriting the app using some more perspective technology.
I need the UI layer/toolkit to be JS-based, which +- means node.js these days. I also require some advanced cross-platform OS integration that you would expect from a file manager; an example would be a way to display the OS's native icon miniatures in a directory/file listing. And the "launch file by the system's default way on doubleclick". You get the idea.
Using HTML+CSS for the UI is preferred, but not strictly necessary.