This is a shorthand attribute for setting the width, style, and/or color of all four edges of an outline around an element in one assignment statement. Attributes that you don't explicitly set with this attribute assume their initial values.

An outline differs from a border in two primary ways. First, an outline does not occupy space in the CSS box model. Rather, the outline simply hovers atop the element, drawn just beyond the border rectangle. Second, CSS does not restrict an outline to be rectangular, allowing an outline to follow the irregular outline of an unjustified paragraph, for example. So far, however, the implementation in IE 5 for the Macintosh draws only rectangular outlines.