Gaia

Gaia is the user interface level of B2G OS. Everything that appears on the screen after B2G OS starts up is drawn by Gaia, including the lock screen, home screen, dialer, and other applications. Gaia is written entirely in HTML, CSS, and JavaScript. Its only interface to the underlying operating system and hardware is through standard Web APIs, which are implemented by Gecko.

Because of this design, not only can Gaia be run on B2G OS devices, but also on other operating systems and in other web browsers (Albeit with potentially degraded functionality depending on the capabilities of the browser).

Third party applications onto the device installed alongside Gaia can be launched by Gaia.