Outline

The outline property is a shorthand property, and sets all three of 'outline-style', 'outline-width', and 'outline-color'.

Description

Values	[ <outline-width> \|\| <outline-style> \|\| <outline-color> ] \| inherit
Initial value	See individual properties
Applies to	all elements
Inherited	No

inherit
Takes the same specified value as the property for the element's parent.

And see also individual properties

[style.css]

 h1 {
   outline: 1px solid red;
 }

[index.html]

  <body>
    <h1>CSS outline property example</h1>
  </body>

The CSS specification defines the outline property in 18.4 Dynamic outlines.