How can I set a css border on one side only?

Question

For a given div I would like to only display a border on the left, right, top, or bottom side.

Currently I have the following, which puts a border on all s

Answer 1

#testDiv{
    /* set green border independently on each side */
    border-left: solid green 2px;
    border-right: solid green 2px;
    border-bottom: solid green 2px;
    border-top: solid green 2px;
}