outline-width

The CSS outline-width property sets the thickness of an element's outline. An outline is a line that is drawn around an element, outside the border.

It is often more convenient to use the shorthand property outline when defining the appearance of an outline.

/* Keyword values */
outline-width: thin;
outline-width: medium;
outline-width: thick;

/* <length> values */
outline-width: 1px;
outline-width: 0.1em;

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

The outline-width property is specified as any one of the values listed below.

<length>

The width of the outline specified as a <length>.

thin

Depends on the user agent. Typically equivalent to 1px in desktop browsers (including Firefox).

medium

Depends on the user agent. Typically equivalent to 3px in desktop browsers (including Firefox).

thick

Depends on the user agent. Typically equivalent to 5px in desktop browsers (including Firefox).

<line-width>
where 
<line-width> = <length> | thin | medium | thick

HTML

<span id="thin">thin</span>
<span id="medium">medium</span>
<span id="thick">thick</span>
<span id="twopixels">2px</span>
<span id="oneex">1ex</span>
<span id="em">1.2em</span>

CSS

span {
  outline-style: solid;
  display: inline-block;
  margin: 20px;
}

#thin {
  outline-width: thin;
}

#medium {
  outline-width: medium;
}

#thick {
  outline-width: thick;
}

#twopixels {
  outline-width: 2px;
}

#oneex {
  outline-width: 1ex;
}

#em {
  outline-width: 1.2em;
}

Result

• outline
• outline-color
• outline-style