Manual:$wgRestrictDisplayTitle

Details
Restricts display titles to titles that normalize to the same  database key as the real page title.

This roughly means that only capitalization variations of the title are accepted.

If set to  , any display title is allowed.

This is not recommended however, as it breaks the wiki convention that a page's title is its name, and thus can be used for linking to it.

Additionally, some inline CSS rules like   which can cause the text to be hidden or unselectable are disallowed.