OOUI

OOUI is a library that allows developers to rapidly create front-end web applications that operate consistently across a multitude of browsers.

The OOUI 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.

OOUI is available as a library in MediaWiki Core as of version 1.23, as well as on npm, Composer and through cdnjs. OOUI implements a "WikimediaUI" theme – conforming to Wikimedia Foundation's user interface style guide. Interfaces built within MediaWiki use this theme by default, though different MediaWiki skins can and do over-ride this. See documentation about using OOUI in MediaWiki.

Note, that the library was originally created for use by VisualEditor, which uses it for its entire user interface, on top of the OOjs primer (Object-Oriented JavaScript, therefore it was former called OOjs UI) and has been later extended to serve MediaWiki Core with PHP implemented widgets