I would rather do it that way :
<body>
Welcome
<div class="DialogMask"></div>
</body>
And for Javascript :
var $loading = $('#Processing');
$loading.hide();
$(document)
.ajaxStart(function () {
$('.DialogMask').show();
$loading.show();
})
.ajaxStop(function () {
$('.DialogMask').hide();
$loading.hide();
});