This property is a shorthand for the following CSS properties:

• outline-color
• outline-style
• outline-width

/* style */
outline: solid;

/* color | style */
outline: #f66 dashed;

/* style | width */
outline: inset thick;

/* color | style | width */
outline: green solid 3px;

/* Global values */
outline: inherit;
outline: initial;
outline: revert;
outline: revert-layer;
outline: unset;

The outline property may be specified using one, two, or three of the values listed below. The order of the values does not matter. As with all shorthand properties, any omitted sub-values will be set to their initial value.

<'outline-color'>

Sets the color of the outline. Defaults to invert for browsers supporting it, currentcolor for the others. See outline-color.

<'outline-style'>

Sets the style of the outline. Defaults to none if absent. See outline-style.

<'outline-width'>

Sets the thickness of the outline. Defaults to medium if absent. See outline-width.

Outline is a line outside of the element's border. Unlike other areas of the box, outlines don't take up space, so they don't affect the layout of the document in any way.

There are a few properties that affect an outline's appearance. It is possible to change the style, color, and width using the outline property, the distance from the border using the outline-offset property, and corner angles using the border-radius property.

An outline is not required to be rectangular: While dealing with multiline text, some browsers will draw an outline for each line box separately, while others will wrap the whole text with a single outline.

Assigning outline a value of 0 or none will remove the browser's default focus style. If an element can be interacted with it must have a visible focus indicator. Provide obvious focus styling if the default focus style is removed.

• How to Design Useful and Usable Focus Indicators
• WCAG 2.1: Understanding Success Criterion 2.4.7: Focus Visible

outline = 
  <'outline-color'>  ||
  <'outline-style'>  ||
  <'outline-width'>  

HTML

<a href="#">This link has a special focus style.</a>

CSS

a {
  border: 1px solid;
  border-radius: 3px;
  display: inline-block;
  margin: 10px;
  padding: 5px;
}

a:focus {
  outline: 4px dotted #e73;
  outline-offset: 4px;
  background: #ffa;
}

Result

• outline-color
• outline-style
• outline-width