1-What is the difference between a class and an ID?
Ans. Class is a way of using HTML elements for styling. They are not unique and have multiple elements. Whereas ID is unique and it can be assigned to a single element. 

2-Define z-index
Ans. Z-index is used to specify the stack order of elements that overlap each other.

3-What is the CSS Box model and what are its elements?
Ans. The CSS box model is used to define the design and layout of elements of CSS.
    The elements are:
    Margin - It removes the area around the border. It is transparent.
    Border - It represents the area around the padding
    Padding - It removes the area around the content. It is transparent.
    Content - It represents the content like text, images, etc.

4-Margin 
 margin: 20px 30px 50px 40px ;
=margin: top  right bottom left;

 margin: 20px         30px;
=margin: top,bottom   right,left;