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

var eOld = null;

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

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