document.onmouseover = TabOver;
document.onmouseout  = TabOut;

var eOld = null;

function TabOver() {
 var eSrc = window.event.srcElement;
  if (eSrc.className == "tab-button2") {
    eSrc.className = "tab-button"; 
  }
}

function TabOut() {
 var eSrc = window.event.srcElement;
  if (eSrc.className == "tab-button") {
    eSrc.className = "tab-button2";
  }
}