An outline is a line that is drawn around elements (outside the borders) to make the element "stand out".

The outline shorthand property sets all the outline properties in one declaration.

The properties that can be set, are (in order): outline-color, outline-style, outline-width.

If one of the values above are missing, e.g. "outline:solid #ff0000;", the default value for the missing property will be inserted, if any.