Giving background-color to body applying whole page. Why?

Answer 1

Body means all page that display in the browser. so you want give color only to div you can use either a id or a class to give a style to that dive. as in first answer.

    
Hello World!


.some{
   background-color: red;
}

or

    
Hello World!


#some{
   background-color: red;
}