Untitled

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="pt" lang="pt-br" >
<head>
	<meta http-equiv="content-type" content="text/html; charset=UTF-8" />
	<title>Teste</title>
	<script type="text/javascript" src="prototype.js"></script>

<script>
	Event.observe(window, 'keypress', 
		function pressiona_tab(evt) {
			if(evt.keyCode == Event.KEY_TAB){
				$("texto").focus();
			}
		});
</script>

</head>

<body>

<form name="legal">
<input type="text" name="texto" id="texto" />
</form>

<a href="#">teste</a>

</body>
</html>