def compare_properties(a, b)
      if a[:property] == b[:property]
        compare_by_vendor(a, b)
      elsif @preferred_order
        compare_by_order(a, b, @preferred_order)
      else
        a[:property] <=> b[:property]
      end
    end
def compare_properties(a, b)
      if a[:property] == b[:property]
        compare_by_vendor(a, b)
      elsif @preferred_order
        compare_by_order(a, b, @preferred_order)
      else
        a[:property] <=> b[:property]
      end
    end
Compares two properties which can contain a vendor prefix. It allows for a
 sort order like:

   p {
     border: ...
     -moz-border-radius: ...
     -o-border-radius: ...
     -webkit-border-radius: ...
     border-radius: ...
     color: ...
   }

 ...where vendor-prefixed properties come before the standard property, and
 are ordered amongst themselves by vendor prefix.