Here is a sample div with class “boxtest”.
It has 20px border, 30px padding, and 300px width.
The total width including borders and padding should be 400px.
20+30+300+30+20 = 400
User agents which misinterpret the CSS1 box model by placing border and padding inside the specified width would result in a total width of only 300px, and a content width of only 200px.
300-20-30-30-20 = 200
The red and blue bars below are there for comparison. This div (including its borders) should be as wide as the blue bar.