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 border properties allow an author to specify the width, color, and style
of the border area (between any specified padding and margins) of an element's
rendering box. While the capability to create simple line effects has been
available in HTML via tables, the CSS border properties give authors much more
power in creating such effects and allows them to be applied to any element.
Each side of the border 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
Official Reference: CSS Level 1, Section 5.5.11-22