Описание тега border-box

This is one of the 3 allowed values for the CSS box-sizing property. When this box model is used, the width and height properties include the padding and border, but not the margin. This is the box model used by Internet Explorer when the document is in Quirks mode.

Example

.example {
  -moz-box-sizing: border-box;
       box-sizing: border-box;
}