The default value for a property, when it’s not specified explicitly or inherited, is called the initial value. The initial value of each property is defined in the CSS specification.

Browsers have user agent style sheets that define the default rendering of the various HTML element types. In some cases, the rules in those built-in style sheets define values other than the initial values from the CSS specification. For example, links are usually underlined, even though the initial value for the text-decoration property is none.

You can read more about these topics in The Cascade, Specificity, and Inheritance.