Everything is horribly broken. At a quick glance, something in the build process is clobbering almost all values measured in 'em', so, almost all values basically.
This happened after I did npm install, and is also visible on https://tools.wmflabs.org/oojs-ui/oojs-ui/demos/, so I blame rGOJUd24df4157db0: build: Updating development dependencies.
This is blocking the next release.