XULRunner

XULRunner is a Mozilla runtime package that does not include a browser. It allows developers to implement applications as rich and flexible as the Firefox web browser or Thunderbird email reader. GECKO is actually the subset of XULRunner that takes care of rendering HTML. More details on what is included are here. Note that the lightweight SQLite relational database is integrated with XULRunner.

XULRunner also supports Javascript however this is not part of GECKO and therefore is not supported by the current Sugar web browser which is a Python application that uses the geckoembed module.