關於權重的計算，在W3C的網頁上的介紹是：
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
翻譯一下
- id的數量為a（ex. #main #sidebar {…} 這樣是2）
- class, attr, 偽類別的數量為b
- tag跟偽元素的數量為c（ li:hover:before {…} 這樣是3）
- 星號直接無視