I just noticed that user CSS preview is broken on my wiki running svn trunk. No <style> element containing the CSS to be previewed seems to be generated. This happens on both Vector and MonoBook, as well as on my custom MonoBook-derived skin (branched about a year ago). User JS preview, however, seems to work fine.
I have not checked if this bug is present in the 1.17 branch too, but if so, it should be fixed before release.
Version: 1.18.x
Severity: normal