In CSS, the fundamental visual rendering model places all components of the
document tree in physical and virtual rectangular boxes, each having a specific
height and width. An element's rendering box consists of an element's content
at the center (text, images, etc.) Surrounding the element's content (moving
outward in rectangular layers/strips) are optional padding, surrounded by any
optional border effects, surrounded in turn by any optional margin values that
may be specified.
The padding properties allow the author to specify how much space will
be inserted between the element border and the element content. Negative
values are not allowed.
Each side of the padding dimensions (top, right, bottom and left) can be
addressed and controlled independently using separate properties, or a
convenient shorthand notation may be used that controls multiple sides at once.
Official Reference: CSS Level 1, Sections 5.5.6-10