The margin-bottom specifies the bottom margin of an element. It can have a value in length, % or auto. You can try to run the following code to set the bottom margin:
Example
<html> <head> </head> <body> <p style = "margin-bottom: 20px; border:2px solid red;"> This is a paragraph with a specified bottom margin . </p> <p style = "margin-top: 20%; border:2px solid green;"> This is another bottom with a specified top margin in percent </p> </body> </html>