Calculating CSS Selector Weight
Posted on 05/20/2008 @ 03:12PMWhen working with complicated stylesheets, I'm always forgetting the rules regarding which rule overrides another when both affect the same element.
Excerpted from the W3C CSS2.1 Specification:
A selector's specificity is calculated as follows:
- count 1 if the declaration is from is a 'style' attribute rather than a rule with a selector, 0 otherwise (= a) (In HTML, values of an element's "style" attribute are style sheet rules. These rules have no selectors, so a=1, b=0, c=0, and d=0.)
- count the number of ID attributes in the selector (= b)
- count the number of other attributes and pseudo-classes in the selector (= c)
- count the number of element names and pseudo-elements in the selector (= d)
The specificity is based only on the form of the selector. In particular, a selector of the form "[id=p33]" is counted as an attribute selector (a=0, b=0, c=1, d=0), even if the id attribute is defined as an "ID" in the source document's DTD.
Concatenating the four numbers a-b-c-d (in a number system with a large base) gives the specificity.
The example makes it far clearer in my opinion:
* {} /* a=0 b=0 c=0 d=0 -> specificity = 0,0,0,0 */ li {} /* a=0 b=0 c=0 d=1 -> specificity = 0,0,0,1 */ li:first-line {} /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */ ul li {} /* a=0 b=0 c=0 d=2 -> specificity = 0,0,0,2 */ ul ol+li {} /* a=0 b=0 c=0 d=3 -> specificity = 0,0,0,3 */ h1 + *[rel=up]{} /* a=0 b=0 c=1 d=1 -> specificity = 0,0,1,1 */ ul ol li.red {} /* a=0 b=0 c=1 d=3 -> specificity = 0,0,1,3 */ li.red.level {} /* a=0 b=0 c=2 d=1 -> specificity = 0,0,2,1 */ #x34y {} /* a=0 b=1 c=0 d=0 -> specificity = 0,1,0,0 */ style="" /* a=1 b=0 c=0 d=0 -> specificity = 1,0,0,0 */
<HEAD> <STYLE type="text/css"> #x97z { color: red } </STYLE> </HEAD> <BODY> <P ID=x97z style="color: green"></P> </BODY>
