Code:
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
<link href="stylesheet.css" rel="stylesheet" type="text/css">
<title>Client Validation</title>
<head>

</head>

<body>
		<center>
			<form name="custInfo" id="custInfo" autocomplete="on">
				<fieldset>
				<legend>Required Customer Information</legend>
					<label>First Name:<input id="firstName" name="name" type="text" placeholder="Forename" autofocus></label>
					<label>Surname:<input id="Surname" name="sName" type="text" placeholder="Surname"></label>
					<label>Telephone:<input id="tel" name="telephoneNum" type="tel" placeholder="07123456789"></label>
					<label>Email Address:<input id="emailer" name="email" type="email" placeholder="email@email.com"></label>
					<br />
					<input type="button" value="Submit!" onclick="isValid()" />
				</fieldset>
			</form>
		</center>
<script src="javascript.js" type="text/javascript"></script>
</body>
</html>