Pau Giner has updated his RfC https://www.mediawiki.org/wiki/Talk:Requests_for_comment/Grid_system with more detail, and has submitted a patchset to Gerrit https://gerrit.wikimedia.org/r/#/c/125387/ so that the discussion can get more specific. Also see the example implementation http://pauginer.github.io/agora-grid/. Please comment!
Excerpts:
There are many kinds of grids with different properties. Considering the MediaWiki context, the most relevant properties considered for a grid system are: Optional, Nestable, Proportional, Responsive and Semantic....
The proposed grid system uses style classes following the breakpoint-proportion format (e.g., desk-two-thirds, palm-one-half) to indicate the layout of elements in a specific screen size....
The grid system will be supported as a set of LESS mixins and variables. This means that, the grid classes will not be exposed to the generated CSS. That will (a) lead to smaller and more semantic CSS and HTML, (b) reduced dependency with the grid concepts used and their implementation, and (c) avoid grid classes to be used out of their intended target (UI as opposed to user-generated content)....
Thanks, Pau!