'property: 0' or 'property: 0px' in CSS?

Question

I've seen this notation used a lot, and I was wondering, is there is any notable difference between these two notations?

element#id {   property: 0; } 

and

element#id {   property: 0px; } 

I use property: 0px; all the time, as I find it cleaner looking, but I'm not really sure if the browser interprets 0px differently than 0.

Does anyone know which one is better or correct?

Answer 1

Unit identifiers are optional, but there is no noted performance increase (although you are saving two characters).

CSS2 - From W3C CSS 2.1 Specification for Syntax and basic data types:

The format of a length value (denoted by <length> in this specification) is a <number> (with or without a decimal point) immediately followed by a unit identifier (e.g., px, em, etc.). After a zero length, the unit identifier is optional.

(Emphasis mine)

CSS3 - From W3C CSS Values and Units Module Level 3 (Currently in Candidate Recommendation at the time of this writing)

For zero lengths the unit identifier is optional (i.e. can be syntactically represented as the 0).

Answer 2

While the unit is optional when the value is 0, I tend to leave it in, as I can then tweak the values with Chrome's Developer Tools by clicking on the value and pressing the up/down arrow keys. Without a unit, that isn't really possible.

Also, CSS minifiers strip the units off of 0 values anyways, so it won't really matter in the end.