This is the {{fmbox warning ruler}} template.

It takes no parameters and produces a horizontal ruler in the same colour as the red border for the {{fmbox}} warning type and the ".fmbox-warning" CSS class in MediaWiki:Common.css.

It is used to separate parts of some interface messages, such as MediaWiki:Cascadeprotectedwarning. See what links here for the rest of the interface messages that use it.

Note: MediaWiki doesn't parse and convert HTML wikimarkup in interface messages. Thus this template must be fully XHTML compliant. This means that any HTML tag in it must have a proper ending. For instance <hr> is wrong, but <hr /> is right.