pseudo selector css

A pseudo-class is a selector that selects elements that are in a specific state, e.g. they are the first element of their type, or they are being hovered over by the mouse pointer.element:nth-child(1)
.element:visitedbutton:hover {
  color: blue;

A pseudo-class is a word such as :hover that makes its changes
to the selected element by adding it along side the selector 
before the curly braces in CSS instead of inside the curly 

You can search for a list of pseudo-class online.

button/anchor:hover / :active / :checked
:nth-of-type(3) // select the third element
:nth-of-type(3n) // select an element every three

Also in JavaScript: