Manual:$wgSiteNotice

Details
This string will be displayed at the top of each page if no sitenotice is defined at MediaWiki:Sitenotice. MediaWiki:Sitenotice takes precedence over this option.

In version 1.5 and version 1.9 onwards, $wgSiteNotice can contain Wiki markup code ("WikiCode").

In version 1.6 through 1.8, WikiCode in the SiteNotice was not rendered, the SiteNotice was treated as raw HTML. This had no effect on sitenotices added to MediaWiki:Sitenotice.