1.specify a state or relation to the selector
selector:pseudo_class { property: value; }
2.Link
3.Dynamic Pseudo Classes
apply styles when something happens to something.
active:when a link is clicked on
hover:when a cursor moves over a link
focus:when it is selected by,is ready for,keyboard input 支持度不太广泛
4.first-child
CSS3；last-child ,target first-of-type