Board Thread:Support Requests - Designing Your Wikia/@comment-2177054-20160607140816/@comment-24473195-20160607151331

The message says it all. They deliberately blocked it because it is possible to abuse the "background-url", and cause security issues. See :

http://security.stackexchange.com/questions/54055/xss-with-style-attribute-background-image

https://www.owasp.org/index.php/XSS_Filter_Evasion_Cheat_Sheet