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.