Extension:Widgets - MediaWiki
www.mediawiki.org/wiki/Extension:Widgets
From the page
Saved with the link. Notes from 2007 to 2016 are mostly the page's own words, so this is probably an excerpt.
The Widgets extension allows the creation of raw HTML pages that can be embedded (similarly to templates) in normal wiki pages. You do this by creating pages in the Widget namespace. They avoid the security problems of raw HTML in editable wiki pages because the privilege to edit in the Widget namespace is managed. The Widgets extension has become the generally-accepted means of embedding raw HTML in MediaWiki, and many pre-written Widgets are available.