The code `outline: ;` is a CSS property declaration with...

The code outline: ; is a CSS property declaration with a syntax error. It tries to define the outline property but does not provide a valid value. For it to be valid, it must specify a value for the outline property, such as none, solid, or a combination like 1px solid red.

As it stands, outline: ; does nothing and will not apply any styles because it is incomplete and invalid CSS.