Sony uses ADE (Mobile reader) as do many devices like my PEz.
It forgives errors, NOT.
Go down to the EPUB forum and read the sticky ADE bugs http://www.mobileread.com/forums/showthread.php?t=94733
(the behavior is a bug, because CSS errors are to be ignored, not cause a crash)
forget a semicolon and your whole CSS will/can be ignored (and it is real fun to find
without using the validator service