User blog comment:DaNASCAT/Update Your CSS and JavaScript To Speed Up Your Wiki/@comment-3508190-20120901063931/@comment-166269-20120906074449

It seems the problem here was actually a missing closing bracket in OldInfoboxStyle.css; when browsers hit this they didn't know how to handle it so they just ignored the @imported CSS. Changing the @import to instead call Oldinfoboxstyle.css fixed the problem only because that file doesn't exist, and since ResourceLoader couldn't find it, it was skipped; it didn't cause (obvious) visual breakage because its contents are an older version of some of w:c:enanimanga:MediaWiki:Anime-Common.css, which picked up the slack. So, no bug here on ResourceLoader's end, just some less-than-perfect copy-pasting a couple of years ago.