content.css, elements.css & interface.css in resources/src/mediawiki.skinning/ are not always ideal:
[21:12:54] <Isarra> I've generally found I never want any of them because they all come with useless (and poorly-written) baggage.
[21:14:03] <Isarra> What I'm proposing, though, is barely even interface. Where those all have styles (borders, backgrounds, sizes, whatever), this is just really simple things like clears and noprints and make RTL actually work and anything else ANY skin, no matter the style, will need and use.
Furthermore shared.css and/or oldshared.css from resources/src/mediawiki.legacy/ are always available on most/all skins (?) and they contain some styles, such as the visualClear class definition, which should ideally be in this new RL module.