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

    div{
    border-left:solid red 3px;
    border-right:solid violet 4px;
    border-top:solid blue 4px;
    border-bottom:solid green 4px;
    background:grey;
    width:100px; height:50px
}

DEMO