What value should I use to change the border of an element back to its default? I tried 'null' and 'undefined' to no avail. When I inserted window.alert(targetControl.borderColor); it returned 'undefined' as the value.

function highlightControl(ctl, mode) {
var targetControl = document.getElementById(ctl);
if (mode == "on") {
targetControl.style.borderColor = "red";
}
if (mode == "off") {
targetControl.style.borderColor = null;
}
}