This is a wish list for HIG pages, it is not a task about finding/discussing possible solutions! It is just a wish list.
Consider this MW skin that uses Zurb Foundation framework to offer a responsive layout: https://www.mediawiki.org/wiki/Skin:Pivot
Demo site: https://pivot.wikiproject.net/wiki/Main_Page
Edit: it seems the older Foundation-using skin called Foreground is still being developed, so it would be good to consider it as well:
https://github.com/thingles/foreground
https://www.mediawiki.org/wiki/Skin:Foreground
Demo: https://foreground.wikiproject.net/wiki/Main_Page
- support for mobile devices ( responsive page layout, ...)
Why is there a need for "responsive page layout"? Are we expecting developers to be coding via their phones?
Responsive layout today is a must. Sites without it get penalized by search engines, for example. Personally I don't own a mobile device capable of browsing the web, but I build all my sites mobile-first responsive (since ~2013). Please trust web design professionals in this.
HIG are not only for developers, but for designers, UI/UX, ... too. A lot of designs does not happen on a desktop pc. And even if you code on a desktop, some devs like to keep reference websites in a very small browser next to there IDE or like for example KDevelop, the IDE has an inbuilt documentation viewer (webbrowser).