Property Reference - CSS Reference - page 226

Name
outline-offset
Values:
<length>
Initial value:
0
Applies to:
All elements
Inherited:
No
Computed value:
An absolute length value
Description:
Defines the offset distance between the outer border edge and inner outline edge. Only one
length value can be supplied and it applies equally to all sides of the outline. Values can be
negative, which causes the outline to “shrink” inward toward the element's center. Note that
outline-offset cannot be set via the shorthand outline .
Examples:
*[href]:focus {outline-offset: 0.33em;}
form:focus {outline-offset: −1px;}