"Since, to some extent, the properties have overlapping functionality, the order in which the rules are specified is important.
Consider this example:
border: solid red;
In the above example, the color of the left border is black, while the other borders are red. This is due to 'border-left' setting the width, style, and color. Since the color value is not given by the 'border-left' property, it will be taken from the 'color' property. The fact that the 'color' property is set after the 'border-left' property is not relevant."
This example comes entirely from CSS 2.1, section 8.5.4