SlavaP,
<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
</head>
<body>
<script type="text/javascript">
function go() {
var pas = "123";
if (document.getElementById("myPas1").value == pas) {
document.getElementById("next").removeAttribute('disabled')
} else {
document.getElementById("next").setAttribute('disabled', 'disabled')
};
};
</script>
<table border="0" cellpadding="0" cellspacing="0" align="center">
<tr>
<td>
<input type="text" size="10" value="" placeholder=" введи 123" name="myPas1" id="myPas1" oninput="go();">
<input type="submit" value="Далее" disabled="disabled" id="next">
</td>
</tr>
</table>
<center>
<div id="da1" style="display:none;">Можете продолжить!</div>
</center>
</body>
</html>