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

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 sides:

#testdiv {
   border: 1px solid;
}

What do I need to do in order to have a border only on the left side?

Solution 1:

#testdiv {
   border-left: 1px solid;
}

See the MDN documentation on border.

Solution 2:

If you want to set 4 sides separately use:

border-width: 1px 2em 5px 0; /* top right bottom left */
border-style: solid dotted inset double;
border-color: #f00 #0f0 #00f #ff0;

Solution 3:

    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