In general, Fandyllic is correct. You should provide a link to your wiki; preferably to your actual test that isn't working. However, in this case, the answer is clear from what you posted. MediaWiki passes all page content through a parser that interprets the content via its own markup language. This markup language happens to allow SOME direct HTML/CSS but not all HTML/CSS. In-line use of CSS's URL function is one of the things that is NOT ALLOWED.