Rejoice, for your admins will feel more native to your surrounding computing environment and likely load faster, especially when offline, as they no longer have to talk to The Google Overlord.
At the time of introduction in 3.8, there were not good system fonts common to all platforms at the time. In the years since, Windows, Android, OS X, iOS, Firefox OS, and various flavors of Linux have all gotten their own (good) system UI fonts.
There will definitely be visual bugs, mainly around alignment and spacing; these should be documented and reported on the ticket and fixed more atomically so that our current and future selves have a better understanding of what happened and why.
The style remains registered, as it is almost certainly in use by themes and plugins.
props mattmiklic.
see #36753.
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Helen Hou-Sandí
Andrew Ozz
Konstantin Obenland