http://www.w3.org/TR/css3-ui/#outline-offset0

[#p01] This is a paragraph containing a span whose outline has been offset.

[#p02] This is a paragraph containing a span whose outline has been offset.

[#p03] This is a paragraph containing a span whose outline may have been offset even though the specification forbids it. If it is two pixels, orange, dashed, or offset inward, then the browser has gone beyond the specification.