So far problems have been reported on 5 sites: SFF, Mi Yodeya, Christianity, Electrical Engineering, and Skeptics. What those sites all have in common, and what turned out to be the culprit the first time this happened on one of those sites, is the lack of base CSS. There are currently nine sites listed as not having the base CSS updates yet:
- Photography
- Wordpress Development
- Role-playing Games
- Electrical Engineering
- Physics
- Science Fiction and Fantasy
- Skeptics
- Mi Yodeya
- Christianity
I suspect that a recent change was pushed that, unknown to its author, depends on the base CSS being present. Last time it was the image uploader. I've no idea what it might be this time, but with luck those with access to the revision logs can narrow it down. I've linked the earliest reports (of the current iteration) that I know about in case their timestamps are helpful.