Back in the 2011 WD, there was a set of "serialization principles", giving general guidance about how to select the correct serialization of a CSS property. In particular, it gave guidance about how to order re-orderable values (use the grammar order) and how to omit omittable values (do so whenever it wouldn't change the result of reparsing, unless it would leave you with an empty value). The CSSWG still refers to and understands these principles as a general "shortest form" idea.

However, that no longer seems to be present in the current draft. In particular, it's missing from the following 2013 WD. This, or something like it, needs to be reinstated so as to actually give some guidance about how to serialize property values.

(We obliquely refer to this principle in the note at the end of Align's baseline section, and dbaron requests a link to the principle we're alluding to in #1415.)