(I couldn't find a list for "Frontend standards group" and I'm not sure if all its engineers are on the design list.)

I congratulated James_F on IRC for VisualEditor/ OOjs UI adopting the mediawiki.ui style, but then it went topsy-turvy:

spagewmf I missed that VisualEditor is now using the mediawiki.ui  style formerly known as "Agora"
James_F No, we are not.
Agora and MediaWiki.ui are both previous designs. This one is different, apparently.
Don't ask me how. :-) It's mostly similar, at least.
It's the MediaWiki Theme for OOjs UI.

spagewmf  How is WMF keeping the "simple" mw-ui-button mw-ui-constructive and OOjs UI's more elaborate oo-ui-widget > oo-ui-buttonElement oo-ui-flaggedElement-constructive  CSS in sync?  Are we able to share LESS rules?
James_F  MW UI isn't being kept in sync with the new design, I believe. I don't think it's credible to try to marry the technologies together with shared files – it'd be a huge amount of work for a short-term hack.

spagewmf  The mw-ui- classes are a lot easier for humans to grok/reuse, e.g. https://en.wikipedia.org/wiki/Template:Clickable_button  It could be very cool for pages to have OOjs UI gadgets someday.
James_F  Well, tough. :-) MW-UI won't be around for much longer.

How soon is "short-term" and "not much longer"? It's less than a year since mediawiki.ui.button made it into the default set of page modules and the Living Style Guide appeared. Are devs going to continue working on  $wgUseMediaWikiUIEverywhere to deliver UX consistency?

I understand the approach espoused by the Frontend standards group  is "use OOjs UI components" and I'm excited to see Bartosz prototyping the UserLogin form in oojs-ui/php. But it'll be a long time before the 15 extensions using mediawiki.ui have all transitioned to OOjs UI.