Manual:$wgUserHtml

Details
Enables/disables the use of limited user-specified HTML in wiki pages. It will be run through a whitelist for security. Set this to false if you want wiki pages to consist only of wiki markup. Note that replacements do not yet exist for all HTML constructs.

This would most probably disabled in a future version. You should probably never set it to false cause it is unmaintained. See bug #8590

This parameter as been removed following the bug report 8590. The change can be seen in 21285 and should be in mediawiki 1.10.