Originally Posted by Valloric
"background: url..." is valid, but I'll have to check the CSS specs to see if whitespace is allowed between the colon and the "src".
Actually, as cuthbert19 says, the spec lists "background-color", but not "background" (or "background-image"), which means it is not supported. Although it is strictly not an error, because in CSS unsupported properties are just ignored, so, from an ePUB perspective, it is like a harmless comment (except that it might be recognized by some readers).
As for the space, the CSS spec says:
A declaration is either empty or consists of a property name, followed by a colon (:), followed by a property value. Around each of these there may be white space.