The CSS selector of interest.
The specificity of the selector.
The selector is invalid.
This is based on https://www.w3.org/TR/css3-selectors/#specificity
A larger number represents a more specific selector.
The smallest possible specificity is 0.
The result is represented as a hex number 0x<aa><bb><cc> where
each component is the count of the respective selector clause.
If the selector contains commas, only the first clause is used.
The computed result is cached, so subsequent calculations for the same selector are extremely fast.
Calculate the specificity of a single CSS selector.