Seems that we did something quickly for the A/B test, and didn't clean/improve the code before pushing to production.
We need to fix these lines:
https://github.com/wikimedia/portals/blob/af01e3bf5850317b60ee6e1ec55125a767acc373/dev/wikipedia.org/assets/postcss/_pab2.css#L53-L58
Thankfully most browsers are not impacted by this bug, because they will read the data URI first.
See support for Data URIs: http://caniuse.com/#feat=datauri