Normally, browsers impose strong restrictions for cross-domain interaction through the Web browser. A certain Web page can make a user browse to a different domain. However, it may not read the content of the retrieved page.... In IE these restrictions ... are broken when it comes to CSS [cascading style sheet] imports. I call this attack CSSXSS or Cascading Style Sheets Cross Site Scripting.
— Matan Gillon
BrowserCertainCross