I’ll just quote his description:
OS.js started out as a tool to use on my home server so i could run GUI applications to configure services without installing X and a window manager (including VNC or similar remote management) and connect from anywhere in the world just using a web-browser. But lately I’ve also found some other uses for it, mainly HTML5 application development.
Includes a window manager and a simple desktop environment (similar to most Linux envs out there). Standards are inspired by GTK and freedesktop.org.
Applications are developed using Glade Interface Designer (GTK+3 XML) and (optionally) an SQL database XML-scheme.
A Virtual Filesystem(VFS) is included. It works with local files, block devices, network connections, compressed files etc.
Applications uses a public API that consists of simple OS operations and calls to the application server-side script and the core libraries (File-system (with restrictions), Networking, Services, Configuration files etc.).
OS.js’ code-base is light-weight and small in size.
CSS: < 200 kb (minimized, including applications etc.)
XML: < 300 kb (including applications etc.)
PHP: < 200 kb
You can find more information at: http://anderse.wordpress.com/os-js/