properties - '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).