Singular properties: 'fn', 'n', 'bday', 'tz', 'geo', 'sort-string', 'uid', 'class', 'rev'. For properties which are singular, the first descendant element with that class SHOULD take effect, any others being ignored.
All other properties MAY be plural. Each class instance of such properties creates a new instance of that property.
url may be plural, so you can have multiple url elements in an hCard.