I've started an RFC page for extending CSS within ResourceLoader.
https://www.mediawiki.org/wiki/Requests_for_comment/ResourceLoader_CSS_Exten...
The general idea is to allow css-like extensions to the css within ResourceLoader to make writing it much more sane. Things like nesting support, implicit vendor prefixes, and a method for site css to use uploads in a sane way.
Some parts of the idea also pave the way to making it possible to safely allow css to be embedded securely into arbitrary WikiText and edited by any user. Making it possible to have sane page layouts which can be drafted in easy ways and also have infoboxes without horrid inline styles or admin privileges.
The talkpage is also open for any ideas on other extensions to css we could make.