OOUI

OOjs UI (Object-Oriented JavaScript – User Interface) is a library that allows developers to rapidly create front-end web applications that operate consistently across a multitude of browsers.

The OOjs UI library contains
 * a set of ready-to-use widgets, layouts, and windows that can be instantiated directly or easily extended,
 * a server-side component that generates compatible output in PHP for cases where JavaScript is not supported,
 * elements that can be easily mixed and matched to create custom user interfaces.

OOjs UI is available as a library in MediaWiki Core as of version 1.23, as well as on npm, Composer and through cdnjs. The library was originally created for use by VisualEditor, which uses it for its entire user interface. In MediaWiki 1.25 OOjs UI implements a "MediaWiki" theme closely aligned with the MediaWiki UI CSS appearance. Interfaces built within MediaWiki use this theme instead of the Apex theme by default, though different MediaWiki skins can and do over-ride this. See documentation about using OOjs UI in MediaWiki.