Manual:$wgUserHtml

Details
Enables/disables the use of limited user-specified HTML in wiki pages. By default this limited HTML is enabled, and to switch it off (setting $wgUserHtml=false) actually causes all kinds of problems throughout the mediawiki interface. As a result, this config variable has been removed i.e. it is no longer possible to switch it off. See bug #8590 The change can be seen in 21285 and should be in MediaWiki 1.10.

The original idea was that you could set this to false if you wanted wiki pages to consist only of wiki markup. Now this is not possible, and there is always some limited HTML permitted. All HTML is run through a whitelist for security. Details of what HTML is permitted, can be found at meta:Help:HTML in wikitext