From http://www.w3schools.com (Copyright Refsnes Data)

CSS Outlines

The CSS outline properties is used to draw a line around an element, outside the border edge.

Examples

Draw a line around an element (outline) (does not work in IE)
This example demonstrates how to draw a line around an element, outside the border edge.

Set the style of an outline (does not work in IE)
This example demonstrates how to set the style of an outline.

Set the color of an outline (does not work in IE)
This example demonstrates how to set the color of an outline.

Set the width of an outline (does not work in IE)
This example demonstrates how to set the width of an outline.

An outline is a line that is drawn around elements, outside the border edge, to make the element "stand out".

The CSS outline properties sets the outlines around elements. You can specify the style, color, and width of the outline.

Note: Outlines do not take up space, and they do not have to be rectangular.

Browser support: IE: Internet Explorer, F: Firefox, N: Netscape.

W3C: The number in the "W3C" column indicates in which CSS recommendation the property is defined (CSS1 or CSS2).

Property	Description	Values	IE	F	N	W3C
outline	A shorthand property for setting all the outline properties in one declaration	outline-color outline-style outline-width	-	1.5	-	2
outline-color	Sets the color of the outline around an element	color invert	-	1.5	-	2
outline-style	Sets the style of the outline around an element	none dotted dashed solid double groove ridge inset outset	-	1.5	-	2
outline-width	Sets the width of the outline around an element	thin medium thick length	-	1.5	-	2