Как вариант...
<!DOCTYPE html>
<html>
<head>
<!--
<script src="http://code.jquery.com/jquery-latest.js"></script>
<link rel="stylesheet" type="text/css" href="tmp.css" />
-->
<style type="text/css">
.error {
color: red;
}
</style>
<script type="text/javascript">
function send() {
if (document.getElementById('fio').value=='') {
document.getElementById('err_fio').innerHTML='Заполните ФИО';
return false;
};
return true;
};
</script>
</head>
<body>
<form onsubmit='return send();'>
<div>
<label>ФИО <input id='fio' type='text' /></label>
<p id='err_fio' class='error'></p>
</div>
<button type='submid'>Send</button>
<form>
</body>
</html>