<script language="JavaScript">

var highlightcolor="yellow"
var ns6=document.getElementById&&!document.all
var previous=''
var eventobj
//Regular expression to highlight only form elements
var intended=/INPUT|TEXTAREA|SELECT|OPTION/

//Function to check whether element clicked is form element
function checkel(which)
{
  if (which.style&&intended.test(which.tagName))
  {
     if (ns6&&eventobj.nodeType==3)
        eventobj=eventobj.parentNode.parentNode
     return true
  }
  else
     return false
}

//Function to highlight form element
function highlight(e)
{
  eventobj=ns6? e.target : event.srcElement
  if (previous!='')
  {
     if (checkel(previous))
        previous.style.backgroundColor=''
     previous=eventobj
     if (checkel(eventobj))
        eventobj.style.backgroundColor=highlightcolor
  }
  else
  {
     if (checkel(eventobj))
        eventobj.style.backgroundColor=highlightcolor
     previous=eventobj
  }
}

</script>

Prova a cliccare con il mouse sugli elementi del form
<br>
<FORM onkeyup=highlight(event) onclick=highlight(event)>
Campo 1:<INPUT size=25><BR>
Campo 2: <TEXTAREA name=S1 rows=5 cols=31></TEXTAREA>
<BR>
Campo 3:<INPUT type=checkbox><INPUT type=checkbox></SELECT>
<BR>
<INPUT type=button value="Form button">
</FORM>