Containing box set to 1000px.
This section uses box-sizing: content-box
This box is 600px wide. With content-box, only the actual width of the content is used for the width argument. The content box is set to 600px wide. To calculate the total width, you must add the padding, border and margin together.
This is the default method.
This section uses box-sizing: border-box
The width is calculated based on the visible area of the box. This includes the content, padding, and border.
This is new in CSS3 and is the preferred approach because generally more intuitive.