Opera before 10.10 permits cross-origin loading of CSS stylesheets even when the stylesheet download has an incorrect MIME type and the stylesheet document is malformed, which allows remote attackers to obtain sensitive information via a crafted document.
http://websec.sv.cmu.edu/css/css.pdf
http://scarybeastsecurity.blogspot.com/2009/12/generic-cross-browser-cross-domain.html
http://lists.opensuse.org/opensuse-security-announce/2010-08/msg00001.html