what is pseudo class in javascript code example

Example 1: css psedo class

button: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 
braces.

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

Example 2: css pseudo classes

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