Many properties in CSS have a default value and don’t have to be explicitly set in the stylesheet.

For example, the default font-weight of text is normal, but this property-value pair is not typically specified in a stylesheet.

The same can be said about the box model that browsers assume. In CSS, the box-sizing property controls the type of box model the browser should use when interpreting a web page.

The default value of this property is content-box. This is the same box model that is affected by border thickness and padding.