I am a go player. I am not a dev here. Just an ordinary human.
Let me remind you of your fears about ability to exploit everything in a bad manner.
What if bad people will utilize CSS to put porno in background? It is almost impossible “to police”.
Despite I’m strongly against considering an ability for abusing as a reason for taking decisions it is ok to motivate you this way.
And how about PSA: UK users will soon be unable to access the forums or chat? So many possibilities for unsafe content caused by CSS.