Across the network (I've checked SO, MSO, SSE, and MSE), for questions and answers, I see that the entire toolbar on the post editor is blank; no icons are present.
The post preview also does not render.
My console has this error:
Uncaught ReferenceError: MathJax is not defined
which makes me think https://meta.stackexchange.com/a/326710/200582 may be to blame.
Clearing my browser cache did not help.
Tinkeringbell confirms via chat that it's not just me, and that the image upload outage affects chat as well as the main site: https://chat.meta.stackexchange.com/transcript/message/7661235#7661235