Webian Shell is a browser based operating system UI entirely dedicated to using the web.
The whole minimalistic UI is a web browser, the system back button is a browser back button and the task manager is a tab manager. By default the home screen is populated with websites and web apps the user has pinned, but any web page can be used as a home screen. Shell has bookmarks, top sites and browsing history.
Latest mockups of tablet version of Webian Shell 0.3:
Latest mockups of desktop version of Webian Shell 0.3:
Below is a demo video of an early prototype of the tablet UI which was built as a connected devices experiment at Mozilla (the Webian project is not affiliated with Mozilla).