Looking over some bug entries about layout on mobile for some pages, especially Portal pages, reminds me that formatting is not just MediaWiki's responsibility -- it's also the wiki authors' responsibility. And of course since it's a wiki, that includes us. ;)
Starting from https://bugzilla.wikimedia.org/show_bug.cgi?id=32123 and friends I looked over some of the mobile style issues on < https://en.wikipedia.org/wiki/Portal:Literature%3E and its related portals.
The two main issues that stood out: * two-column layouts being too small to fit their content on small screens * hardcoded table layouts, usually of icons of related portals to click on
The two-column layout I found I was able to make fall back to a single-column layout with a little tweaking: moving the hardcoded float & width styles out to a class in MediaWiki:Common.css, and using CSS media queries to disable the width settings and let them become full column on smaller screens.
(The media query right now won't actually get used on mobile, since we don't actually load the MediaWiki:Common.css -- but it has the same effect. :)
I started a discussion on this at < https://en.wikipedia.org/wiki/MediaWiki_talk:Common.css#Portal_column_width_... -- if this is working well we can go ahead and fix up a bunch of portal pages by bot.
For the hardcoded tables of icons, I started on a new template with a more modern layout that simply fits the window, similar to how <gallery> got a facelift a while ago. Links to that template & demo are at the above talk thread.
Don't forget -- MediaWiki extends to and binds with its content, and making our flagship sites look good on large and small screens alike should be all our responsibility. Chip in and help fix any bad layouts you find!
-- brion