Padding is the space between an element's content area and its border. Padding is not transparent, the padding area uses the element's background as its background. Padding width can be specified using absolute or relative units.
Relative units calculate the padding width using the width of the element's containing block .
It is illegal to use negative values for padding widths. You can enter one to four values to set different widths for different sides of the content area. If there is one value, all four sides are the same width. If there are two values, the first value sets the top and bottom padding widths, and the second value sets the left and right padding widths. If there are three values, the first value sets the top padding width, the second sets the left and right padding widths, and the third sets the bottom padding widths.