files/en-us/glossary/gecko/index.md
Gecko is the layout engine developed by the Mozilla Project and used in many apps/devices, including {{glossary("Mozilla Firefox","Firefox")}} and {{glossary("Firefox OS")}}.
Web {{glossary("browser","browsers")}} need software called a layout engine to interpret {{glossary("HTML")}}, {{glossary("CSS")}}, {{glossary("JavaScript")}}, and embedded content (like images) and draw everything to your screen. Besides this, Gecko makes sure associated {{glossary("API","APIs")}} work well on every operating system Gecko supports, and that appropriate APIs are exposed only to relevant support targets. This means that Gecko includes, among other things, a networking stack, graphics stack, layout engine, a JavaScript virtual machine, and porting layers.
Since all Firefox OS apps are Web apps, Firefox OS uses Gecko as its app runtime as well.