If a receiver doesn't respond correctly to +/-3v signalling it is non-compliant. Otherwise it is compliant.
If a driver can't muster +/-5v into a terminated line it is non-compliant. Otherwise it is compliant.
It IS as simple as that.
I doubt if there have been 20% compliant interfaces on personal computers. And serial printers are a bigger offender. Most of these non-compliances, however, result from the standard's origin and the context of DTE and DCE. Which should a PC be?
Indeed.