Design/Living style guide
A living style guide offers improved documentation of the visual appearance and behavior of a site.
- http://tools.wmflabs.org/styleguide/desktop/ for the mediawiki.UI modules in core since release 1.22
Starting with release 1.25, when you use OOUI components within MediaWiki, they use its WikimediaUI theme which has its roots in mediawiki.UI.
OOUI library live demos