As already in place by a few skins like Vector (legacy skin) and needed by task like T277365.
font-size-base is one of the pivotal points for every skin as some skins might (rightfully) choose to use it for interface calculations.
line-heights are necessary for typographic reasons, but also for constructing the correct box sizes in current OOUI/mediawiki.ui based Vector/MF elements while enabling user-preference text sizing overrides.