index.html

<html>
	<head>
		<title>Exemplo</title>
	</head>
	<body>
		<form id="form">
		 	<input id="name" type="text" placeholder="Nome" required/> 
		 	<input id="lastname" type="text" placeholder="Sobrenome" required/> 
		 	<input name="submit" type="submit" value="Enviar" /> 
		</form>
	</body>
	<script type="text/javascript">
		var elements = document.getElementsByTagName("INPUT");
		for (var i = 0; i < elements.length; i++) {
		    elements[i].oninvalid = function(e) {
		        e.target.setCustomValidity("");
		        if (!e.target.validity.valid) {
		            e.target.setCustomValidity("Preencha o campo "+e.target.placeholder);
		            //customize sua mensagem acima
		        }
		    };
		    elements[i].oninput = function(e) {
		        e.target.setCustomValidity("");
		    };
		}
	</script>
</html>