I have often hit up against zero requiring units. A distance of zero is just zero. Apply units to it if you want but it is just as happy without. CSS to its credit allows zero without px or em etc. Most software misses this point. I have had to set up details of a person with little more than their name, then being required to provide a currency for their zero value account. It's like saying a field is empty and being asked what animals it is empty of.