Manual:$wgRawHtmlMessages/en

Details
This variable is primarily meant for extension so they can register sensitive messages. Extension authors should add all messages here which will be displayed without escaping (e.g. messages which use or, and then send the text to the output without further escaping). The list is used for access control, changing messages listed here will require  and   rights in addition to   already required for editing MediaWiki namespace.

To add values in an extension, add a section like this to :

Message names must be given with underscores rather than spaces and with lowercase first letter.