Using Outline in CSS

An outline is sort of a like a border, but there are some very important differences.

1. outlines do not participate in the flow of the document like borders do, and thus don't trigger document reflow as they appear and disappear.
2. outlines can be nonrectangular

Outlines are considered to be part of user interface styling because they are most often used to indicate the current focus.

Outline Property :

Values
[ <> || <> || <> ] | inherit

Initial value
not defined

Applies to
all elements