KaiOS apps are based on web technologies โ€“ HTML, CSS, and JavaScript and are run by Gecko runtime. If you have written a web page you already know the basics.

Rendering is done by parsing HTML/CSS and painted using graphic APIs. JavaScript are executed by the JS engine SpiderMonkey and connected to C++ components by XPConnect and WebIDL bindings. Applications and the core process communicate only through IPC protocols defined by IPDL.

Additionally a set of core components including file and memory management, threads and data structures etc. are provided through the XPCOM component object model.