Puoi fare cosi

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
<script language="JavaScript" type="text/javascript">
<!--
function simola(unOgetto) {
 alert("valore errato");
 unOgetto.focus();
 unOgetto.select();
}
//-->
</script>

</head>
<body>
Immettere del testo ->
<input type="text" id="t0" ></br>
<input type="button" value="Simola" onclick="simola(document.getElementById('t0'))">
</body>
</html>