If you add
.badge-recommendedarticle { list-style-image: url( "https://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Rekommenderad_grön.svg/9px-Rekommenderad_grön.svg.png" ); }
to MediaWiki:Common.css, it's turned into
.badge-recommendedarticle { list-style-image: url("/w/ \"https://upload.wikimedia.org/wikipedia/commons/thumb/c/c7/Rekommenderad_grön.svg/9px-Rekommenderad_grön.svg.png\""); }
The starting "/w/" causes this to fail. This seems kinda dangerous, as https://www.mediawiki.org/wiki/Manual:Coding_conventions/CSS#Whitespace says
One space after a starting and before an ending parentheses (( and )) in selectors (ex. :not()) and properties (ex. rgba()).
ResourceLoader should either be extended to allow leading and trailing spaces, or the style guidelines at mediawiki.org should be revised to no longer recommend this.