Manual:$wgRawHtml

Details
When   the wiki will allow you to insert raw unchecked HTML.

However, you must embed your html within the  ...  tags so that mediawiki can differentiate it.

This is very dangerous on a publicly editable site, because it allows for arbitrary JavaScript code to be inserted, opening the door for session hijacking. Thus, you shouldn't enable it unless you've restricted editing to trusted users only with  (version 1.3.x and 1.4.x can use ). See  for more information on restricting write access. }}

This option does not affect how wikicode outside of   tags is handled. }}

If you have $wgRawHtml turned on and you notice some HTML being stripped out, you may need to turn off . }}

Is enabling raw HTML necessary?
Some HTML tags are permitted in wikitext, even with $wgRawHtml=false</tt></>.

See meta>m:Help:HTML in wikitext</>|m:Help:HTML in wikitext.

The vast majority of fancy formatting seen on Wikimedia sites is achieved using these limited tags (e.g. tables with CSS style tags).

If you can make do with these limitations (leave <tvar|code><tt>$wgRawHtml=false</tt></>), your wiki will be more secure.

Also note that the "limited" wiki syntax is actually a deliberate design feature of wikis.

It is a compact simplified markup which is easily understood even by non-technical users, easily visualised in diff displays, and discourages stylistic tinkering in favor of getting on with writing useful/interesting text.

Related Extensions
There are a number of extensions which promise to allow more HTML flexibility, while improving the security situation.

Some require setting <tvar|code><tt>$wgRawHtml=true</tt></> in conjunction with using the extension, while others offer an alternative.

adds "secret key" protection for html sections. allows HTML on protected pages only allows HTML on protected pages only + namespace controls allows HTML in specific namespaces allows pre-defined HTML snippets with <tvar|code><tt>$wgRawHtml=false</tt></> allows HTML as stored in <tvar|mw><tt>MediaWiki</tt></> namespace only (does not work on MediaWiki 1.18+) allows HTML and Smarty PHP templates, ostensibly in the form of "widgets", but it can do nearly anything else too.

Another way get custom HTML appearing within your wiki articles is to develop your own tag>Special:MyLanguage/Manual:Tag extensions</>|tag extension.

Do not be tempted to develop an extension which allows arbitrary HTML, otherwise the same serious security issues apply as with setting <tvar|code><tt>$wgRawHtml=true</tt></>.