Which of the following is not a part of the CSS box model?

The CSS box model is a fundamental concept in web design that describes how elements on a web page are structured and how their size is calculated. This model consists of several components: the content area, padding, border, and margin. Each of these components plays a role in determining the space an element occupies and how it interacts with other elements.

The background is not considered a part of the box model itself. Instead, the background is a property that can be applied to the entire box, filling the content area, padding area, and border area; however, it does not contribute to the model's structure or sizing of the box. The background property pertains to visual styling rather than the dimensions or spacing of the box model components. Therefore, understanding the box model helps in managing spacing and layout effectively, while the background serves more to enhance the visual presentation of the box.