CSS outline-width

<!doctype html>
<title>Example</title>
<style>
div {
  font-size: 5vw;
  padding: 5vw;
  margin: 5vw;
  outline-width: 5vw;
  outline-style: solid;
  outline-color: orange;
  border: 1vw solid green;
  }
</style>
<div>Orange outline, green border.</div>

The CSS outline-width property is used to specify the width of an element's outline.

Unlike an element's border (e.g. set using border or its associated properties), an element's outline does not take up extra space and it can be non-rectangular.

The outline is always on top of a box, and it does not influence the position or size of the box, or of any other boxes. Therefore, displaying or suppressing outlines does not cause reflow or overflow.

User agents typically render an outline on elements in the :focus state.

Syntax

These values are explained below.

Possible Values

length: Allows you to specify a non-negative length value as the outline's thickness. Negative values are invalid.
thin: Specifies a thin outline, as determined by the user agent.
medium: Specifies an outline of medium thickness, as determined by the user agent.
thick: Specifies a thick outline, as determined by the user agent.

In addition, all CSS properties also accept the following CSS-wide keyword values as the sole component of their property value:

initial: Represents the value specified as the property's initial value.
inherit: Represents the computed value of the property on the element's parent.
unset: This value acts as either inherit or initial, depending on whether the property is inherited or not. In other words, it sets all properties to their parent value if they are inheritable or to their initial value if not inheritable.

General Information

Initial Value: medium
Applies To: All elements
Inherited?: No
Media: Visual

Example Code

Accessibility Considerations

The CSS spec warns against removing the outline on elements in the :focus state:

Keyboard users, in particular people with disabilities who may not be able to interact with the page in any other fashion, depend on the outline being visible on elements in the :focus state, thus authors must not make the outline invisible on such elements without making sure an alternative highlighting mechanism is provided.

Official Specifications

CSS Basic User Interface Module Level 3 (CSS3 UI) (W3C Candidate Recommendation, 02 March 2017)
CSS Level 2.1 (W3C Recommendation 07 June 2011)