Sadly https://www.mediawiki.org/wiki/Agora has no guideline as to when to use mw-ui-big, mw-ui-primary, mw-ui-constructive, etc.

We could split the prescriptive real-world "Do this with current code" from aspirational "We want the experience to be like this" design guide.  But I would rather have a "Status" section on every design guide page anchoriong it to reality, saying what our skins and extensions currently implement and pointing to code in development. I've added this information to pages and others have removed it, probably because it's messy and disorganized; but so is the real world of MediaWiki 1.22wmf13. Maybe the Status section could be hidden by default so the design guide looks good (I'm not being snarky).