This is a shorthand property which allows an author to specify ‘border-top-color’, ‘border-right-color’, ‘border-bottom-color’, and ‘border-left-color’ properties using a single property and value notation (the values are given in this order separated by spaces.) If one or more of the values are not present, the value for a missing side is taken from the opposite side that is present. If only one value is listed, it applies to all sides. If no border-color is specified for an element’s border, the value of the ‘color’ property is used instead.

em { border-color: blue }

this is an emphasized test

Possible Values
Value Description
inherit Explicitly sets the value of this property to that of the parent.
transparent The border is transparent (with any accompanying ‘border-width' thickness) - eg: elements below it shine through.
[color] Sets the border to the indicated color value.