The initial CSS keyword applies the initial value of a property to an element. It is allowed on every CSS property and causes the element for which it is specified to use the initial value of the property.
The revert CSS keyword rolls back the cascade so that the property takes on the value it would have had if there were no styles in the current style origin (author, user, or user-agent). In author stylesheets (the normal case), for the purposes of the given declaration, it's as if there were no author-level styles, thus resetting the property to the default value established by the user-agent stylesheet (or by user styles, if any exist).
The inherit CSS-value causes the element for which it is specified to take the computed value of the property from its parent element. It is allowed on every CSS property.
The unset CSS keyword is the combination of the initial and inherit keywords. Like these two other CSS-wide keywords, it can be applied to any CSS property, including the CSS shorthand all. This keyword resets the property to its inherited value if it inherits from its parent or to its initial value if not. In other words, it behaves like the inherit keyword in the first case and like the initial keyword in the second case.