codice:
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Esempio</title>
<script type="text/javascript">
function onsubmitForm () {

	var aHideEl = document.getElementsByClassName("test_box");

	for (var nLen = aHideEl.length, nIdx = 0; nIdx < nLen; nIdx++) {
		aHideEl[nIdx].style.setProperty("display", "none");
	}

	document.getElementById("loading").style.removeProperty("display");

}

onload = function () {
	document.getElementById("loading").style.setProperty("display", "none");
};

</script>
</head>
 
<body>

<div class="test_box"><form method="post" onsubmit="onsubmitForm();">
<input type="submit" id="submit" name="submit" value="Invia">
</form></div>

<div id="loading">Div da nascondere al caricamento</div>

</body>
</html>