When there is a backslash-semicolon (:) escape sequence in identifier in CSS rule selector, the backslash is removed.
e.g.
fck:rfe-audio
becomes
fck:rfe-audio
That completely changes meaning of the selector.
Originally it selects element named <fck:rfe-audio>. After compression it selects element <fck> which is of pseudo-class (or pseudo-element) "rfe-audio" (which is not defined). Compressed CSS is
considered invalid by W3C validator though before compression it was perfectly valid. And what's worse - the efect in browser is completelly different (FF, CH, OP, SF) - the selector is not matched
and the CSS rules are not applied.

Besides http://www.w3.org/TR/css3-syntax/#characters states: "Style sheet preprocessors must not remove these backslashes from a style sheet since that would change the style sheet's meaning."