From the spec:
A selector's specificity is calculated as follows:
- count the number of ID selectors in the selector (= a)
- count the number of class selectors, attributes selectors, and pseudo-classes in the selector (= b)
- count the number of type selectors and pseudo-elements in the selector (= c)
- ignore the universal selector
Selectors inside the negation pseudo-class are counted like any other, but the negation itself does not count as a pseudo-class.
Concatenating the three numbers a-b-c (in a number system with a large base) gives the specificity.
Examples:
* /* a=0 b=0 c=0 -> specificity = 0 */
LI /* a=0 b=0 c=1 -> specificity = 1 */
UL LI /* a=0 b=0 c=2 -> specificity = 2 */
UL OL+LI /* a=0 b=0 c=3 -> specificity = 3 */
H1 + *[REL=up] /* a=0 b=1 c=1 -> specificity = 11 */
UL OL LI.red /* a=0 b=1 c=3 -> specificity = 13 */
LI.red.level /* a=0 b=2 c=1 -> specificity = 21 */
#x34y /* a=1 b=0 c=0 -> specificity = 100 */
#s12:not(FOO) /* a=1 b=0 c=1 -> specificity = 101 */
