document.getElementById("text-area").onkeydown = function(e) {
          if (!e && event.keyCode == 9)
          {
            event.returnValue = false;
            insertAtCursor(document.getElementById("text-area"), "  ");
          }
          else if (e.keyCode == 9)
          {
            e.preventDefault();
            insertAtCursor(document.getElementById("text-area"), "  ");
          }
        };

        //http://alexking.org/blog/2003/06/02/inserting-at-the-cursor-using-javascript#comment-3817
        function insertAtCursor(myField, myValue) {
          //IE support
          if (document.selection) {
            var temp;
            myField.focus();
            sel = document.selection.createRange();
            temp = sel.text.length;
            sel.text = myValue;
            if (myValue.length == 0) {
              sel.moveStart('character', myValue.length);
              sel.moveEnd('character', myValue.length);
            } else {
              sel.moveStart('character', -myValue.length + temp);
            }
            sel.select();
          }
          //MOZILLA/NETSCAPE support
          else if (myField.selectionStart || myField.selectionStart == '0') {
            var startPos = myField.selectionStart;
            var endPos = myField.selectionEnd;
            myField.value = myField.value.substring(0, startPos) + myValue + myField.value.substring(endPos, myField.value.length);
            myField.selectionStart = startPos + myValue.length;
            myField.selectionEnd = startPos + myValue.length;
          } else {
            myField.value += myValue;
          }
        }