OOjs UI is optimized for font-size of 12.8px (0.8×16px), but sets it inconsistently for windows and toolbars only, requiring CSS counteractions.
We should remove the font-size: 0.8em rules sprinkled throughout OOUI and instead set the right font-size for windows in each skin's CSS (unless the skin already sets a sensible font-size on body, which they all should but seemingly none does). This will be a delicate endeavor that must be synchronized, but it will also remove some technical debt that keeps breaking things (right now VE inspectors in MonoBook are being displayed with incorrect, massive font size, for example).
(This is in preparation to switching the MediaWiki theme to use 0.875em font size.)