<form name="auth-client" method="get" action="check.php" onsubmit="return validateFormClient()">
						<input type="hidden" name="method" value="client">
						<label><span class="label">Пользователь:</span><input type="text" name="username" placeholder="###-####" size="10"><span class="debug tips"></span></label>
						<label><span class="label">Пароль:</span><input type="password" name="password" placeholder="########" size="10"><span class="debug tips"></span></label>
						<label><input type="checkbox" name="terms" value="agreed">Я принимаю условия предоставления услуги</label>
						<button type="submit">Подключиться</button>
					</form>

function checkField(fld,mask)
{
	lbl = fld.?
...
}
function validateFormClient()
{
	if (!checkField(document.forms["auth-client"]["username"],"###-####")) {return false}
	if (!checkField(document.forms["auth-client"]["password"],"")) {return false}
	if (!checkField(document.forms["auth-client"]["terms"],"")) {return false}
	return true;
}