CSS Box Model

          Every element in an HTML document is rendered by a web browser as a rectangular box based on the CSS box model. The CSS box model is a container that wraps around an HTML element. It contains the properties such as borders, margin, padding and the content of the element itself. This model is used to customize the layout of each element.

Figure 2. The box model containing margin, border, padding, content.
Source: https://www.w3schools.com/css/css_boxmodel.asp)