However, NaCl is not a standard, and not available on any browser other than Google Chrome. It is open source, but in many ways it isn't a clearly defined part of the web. Web applications using Native Client can be published to the Google Chrome Web Store.
Qt applications are already widely portable, and can run on Linux, Mac OSX, Windows, Android, and many other systems. GTK , a UI framework that is used by Gnome, has another approach to enabling apps to run in the browser via an HTML5 backend. Since that uses standard canvas elements, it is perhaps a better approach that will work on more browsers.