Cascading Style Sheets: The Box Model

When the browser draws an object on a page, it places it into an invisible rectangular space called a “bounding box.”
You can specify the size, look, and feel of the margins, the padding, the border, and the content of the box.
Internet Explorer interprets CSS box styles differently than most other web browsers.
In CSS1, the width property is defined as the distance between the left and right edges of the bounding box that surrounds the element's content.
Likewise, the height property is defined in CSS as the distance between the top and bottom edges of the bounding box.
In Internet Explorer, however, the width and height properties also include the border and padding belts that surround the element's bounding box.

When the browser draws an object on a page, it places it into an invisible rectangular space called a “bounding box.”