New properties in CSS2 allow outlines to be created for an element to
help it stand out from other elements. CSS Outlines are very similar
to borders except for several subtle differences:
Border sides can be independently controlled, Outline sides can
not. The visual effects for an outline are uniform for all sides.
Outlines may be non-rectangular.
Outlines do not take up space in the box rendering model, they are
rendered on "top" of the element's box. An outline does not affect
the position or size of the rendering box.