Looking at the recent bug fixes, it seems that the way this library works, if anything goes wrong, it causes a fatal error, so you get no CSS at all.
A better strategy might be: Wrap the processing of each rule in a try-catch, and if there is an error, then just leave that rule alone (perhaps insert a CSS comment flagging the problem so it can be debugged - and Moodle coudl scan for those comments, and report them, before minifying the CSS).
Looking at the recent bug fixes, it seems that the way this library works, if anything goes wrong, it causes a fatal error, so you get no CSS at all.
A better strategy might be: Wrap the processing of each rule in a try-catch, and if there is an error, then just leave that rule alone (perhaps insert a CSS comment flagging the problem so it can be debugged - and Moodle coudl scan for those comments, and report them, before minifying the CSS).