I have been unable to get background-image:url( someURL ) to work with this extension, like below:
{{#css:
/*
This code requires the CSS extension (previously used NewPageCSS)
http://www.mediawiki.org/wiki/Extension:CSS
*/
.someClass { background-image:url(http://www.somedomain.com/w/images/0/06/someimage.png); }
}}I believe it is due to Sanitizer::checkCss() as described by Brion here:
http://www.gossamer-threads.com/lists/wiki/mediawiki/233179
The CSS that gets sanitized by the parser is getting stripped out of the CSS extension too.
However, the usual /* insecure input */ error message appears in the <head> of the page source, like this:
<style type="text/css"> /*<![CDATA[*/ /* insecure input */ /*]]>*/ </style>
The only way to get around this problem is to put the CSS in MediaWiki:Common.css.
There are good reasons for the parser to strip some CSS out, but in addition to documenting this issue (which this bug does, and I'll do in the extension docs in a moment), it should be configurable whether the CSS extension lets the parser sanitize, for example, when used on private wikis.