When a stylesheet containing @import is concatenated with another stylesheet, the @import rule breaks because it has to stay at the top.
For core and extensions we do not support usage of @import, however for site and user modules it can't be missed (i.e. @import global.js from meta, @import css from another sub page, @import css for cross-wiki gadgets..).
bug 33305 introduced this breakages for <style> tag in the front-end, that can be fixed. However it turns out this isn't the first time we concatenate style sheets. the load.php for only=styles also concatenates stylesheets.
Although it is never to be used with more than 1 module when dealing with user-generated content, the "user" and "site" are loaded separately but exist of more than 1 source page.
user: User:Name/common.css + User:Name/<skin>.css
site: MediaWiki:Common.css + MediaWiki:<skin>.css
Ever since 1.17 @import rules in the 2nd pages have been broken. In common.css they still work fine.
Version: 1.17.x
Severity: minor