<style>
<!--
label,input {
	display: block;
	width: 150px;
	float: left;
	margin-bottom: 10px;
}

label {
	text-align: right;
	width: 75px;
	padding-right: 20px;
}

p {
	clear: left;
	margin: 0;
	margin-bottom: 10px;
}

span.error {
	display: block;
	float: left;
	margin-left: 20px;
	color: #cc0000;
	font-weight: 600;
}

input.error {
	border: 1px solid #cc0000;
	background-color: #ffffff;
}

-->
</style>

<script>
var W3CDOM = (document.getElementsByTagName && document.createElement);

/***
window.onload = function () {
	document.forms[0].onsubmit = function () {
		return validate()
	}
}
***/

function validate()
{
	validForm = true;
	errorstring = '';
	var x = document.forms[0].elements;
	for (var i=0;i<x.length;i++)
	{
		if (!x[i].value)
			writeError(x[i],'This field is required');
	}
	if (x['email'].value.indexOf('@') == -1)
		writeError(x['email'],'This is not a valid email address');
	if (!W3CDOM)
		alert(errorstring);
	if (firstError)
		firstError.focus();
	return validForm;
}

function writeError(obj,message)
{
	if (obj.hasError) return;
	if (W3CDOM)
	{
		obj.className += ' error';
		obj.onchange = removeError;
		var sp = document.createElement('span');
		sp.className = 'error';
		sp.appendChild(document.createTextNode(message));
		obj.parentNode.appendChild(sp);
		obj.hasError = sp;
	}
	else
	{
		errorstring += obj.name + ': ' + message + '\n';
		obj.hasError = true;
	}
	if (validForm)
		firstError = obj;
	validForm = false;
}

function removeError()
{
	this.className = this.className.substring(0,this.className.lastIndexOf(' '));
	this.parentNode.removeChild(this.hasError);
	this.hasError = null;
	this.onchange = null;
}

</script>

http://www.quirksmode.org/dom/error.html
<BR>

<form name="example" method=post onSubmit="return
validate();">
<p><label for="name">name</label>
<input size=20 name="name" id="name"></p>
<p><label for="address">address</label>
<input size=20 name="address" id="address"></p>
<p><label for="city">city</label>
<input size=20 name="city" id="city"></p>
<p><label for="email">e-mail</label>
<input size=20 name="email" id="email"></p>
<p><input type=submit value="Submit form"></p>
</form>